Heredity

What is a barr body?

A barr body is the dark and coiled chromatin structure of the X chromosome that is inactivated. This is an important concept because if the female is a carrier of an X-linked recessive disorder, and her unaffected X chromosome gets inactivated, then she would become an affected individual expressing the disorder.

What is a dihybrid cross in short?

A dihybrid cross is a cross between individuals with two genes. In such a cross, we start off with our F1 offspring as AaBb.

What is a gamete and what does it include?

A gamete (haploid cells of 23 chromosomes) includes eggs and sperm.

What is a gene?

A gene is a specific sequence of DNA that codes for a trait.

What is a locus?

A locus is the location of a gene on a chromosome.

What is a monohybrid cross?

A monohybrid cross examines the pattern of inheritance for a single (mono = one) gene when two heterozygous individuals are crossed. We can see that for the F2 generation, the genotype ratio (YY:Yy:yy) should always be (1:2:1). The phenotype ratio (dominant:recessive traits) should always be (3:1).

What is a mutation?

A mutation is a change to DNA that will be passed to its daughter cells - i.e., it isa heritable change to the DNA.

What is a test-cross (or backcross) and when so we use this type of cross?

A test-cross is used to determine an unknown parental genotype, based on the phenotypes seen in the offspring generations. In a test-cross, one parent of an unknown genotype will be bred with a parent that is phenotypically recessive with genotype homozygous recessive (aa). A testcross can be used for an organism that shows dominance, but we are unsure if it is homozygous or heterozygous. Let A = red color Let a = white color We must cross it with a homozygous recessive individual: If we have A_ and we want to tell if it was AA or Aa, we cross it with aa (homozygous recessive) and note the following: (see diagram) If we get all red...our original was AA If we get a 50-50 mix...our original was Aa.

What is a true breeding organism? What offspring would be expected from a true breeding parent and a test-cross?

A true breeding organism is homozygous for its traits (e.g. AA or aa). What offspring would be expected from a true breeding parent and a test-cross? If one parent is homozygous dominant and the other one is homozygous recessive, then their offsprings will be all heterozygous (Aa).

What do acetylated histone proteins do?

Acetylated histone proteins will form euchromatin, which is easy to transcribe. This leads to genetic activation.

Sometimes genetic testing can be done on a fetus to find out if any genetic abnormalities are present. These include what?

Amniocentesis, Ultrasound, and CVS (Chorion Villus Sampling)

What does autosomal mean?

Autosomal means the genes are not on either of the two sex chromosomes (X or Y). There are 44 autosomal chromosomes, and they do not have a sex linkage.

What are chromosomes?

Chromosomes, located in the nuclei of cells contain the hereditary information that will direct the synthesis of thousands of proteins. All cells except sperm and ova are diploid...meaning there are 23 pairs of chromosomes or 46 total chromosomes in humans.

What does complete penetrance mean in heredity?

Complete penetrance is when the trait is expressed in the full population that has the allele. If everyone with a dominant brown eye allele expressed brown eyes, brown would be completely penetrant for the eye color gene.

One mechanism that can affect a person's epigenetics is called DNA methylation. Explain this method.

DNA methylation is a method of gene suppression. During the methylation of DNA, some nucleic acids will have their hydrogen atoms removed and replaced with a methyl group. This leads to gene suppression by recruiting methyl-binding proteins (MBDs) and preventing transcription factors from binding to DNA.

One type of syndrome aneuploidy causes is down syndrome, explain this syndrome.

Down syndrome is trisomy at chromosome 21, resulting from nondisjunction at chromosome 21, producing a zygote with 2n+1 (47) chromosomes. Down syndrome is a trisomy that is non-fatal, meaning that this altered chromosome number does not kill the embryo. In down syndrome, symptoms include mental retardation, heart defects, and individuals are more prone to Alzheimer's and Leukemia. 95% of non disjunction cases resut into Down Syndrome.

What does each chromosome contain?

Each chromosome contains DNA segments that code for the basic units of heredity and are transmitted from one generation to the next. These DNA segments are called genes. Each gene has its own location on a chromosome called a gene locus.

What does expressivity mean in heredity?

Expressivity refers to the degree of a certain phenotype. For example, imagine that the genotype for hair is Hh. We know that hair quantity, thickness, and style varies from person to person. This means that even if everyone has the genotype Hh, the phenotype expressivity would be different for every person with hair.

During which phase in meiosis I and II does non-disjunction occur in?

Failure of chromosomes to separate in Anaphase I and Anaphase II of meiosis.

What are linkage maps?

From this knowledge, scientists can draw out linkage maps. Linkage maps do not give a physical distance between linked genes; rather, they use map units to infer the distance between genes on a chromosome. The greater the map units, the further apart the genes.

What is a gain-of-function mutation?

Gain-of-function mutations cause an increase in gene expression, or an increase in gene-product activity (an over-active protein). A gain of function mutation in proto-oncogenes can produce oncogenes (genes that can transform a normal cell into a cancerous cell).

What is genomic imprinting?

Genomic imprinting means that the way the gene is expressed depends on which parent the gene came from. It is influenced by epigenetic modifications. Less than 1% of genes exhibit genomic imprinting. For example, certain genes that code for an insulin growth factor are only expressed on the gene of paternal origin.

One type of gene defect is haploinsufficiency, explain this.

Haploinsufficiency means that one copy of a wild type gene is not sufficient to achieve a normal phenotype. In other words, the normal trait requires two functional gene copies, because one copy will not produce enough protein to show the trait.

One type of gene defect is haplosufficiency, explain this.

Haplosufficiency means that one copy of a wild type gene is sufficient to achieve a normal phenotype. In other words, the normal trait only requires one functional gene copy because it produces enough protein to show the normal trait.

What is heredity?

Have you ever wondered why family members tend to look alike, or why certain disease run in families? It all has to do with heredity, which is the passing of traits from parents to offspring.

What does histone de-acetylation do?

Histone de-acetylation forms heterochromatin, which is difficult to transcribe. This leads to genetic suppression.

What does histone methylation do?

Histone de-acetylation forms heterochromatin, which is difficult to transcribe. This leads to genetic suppression.

How can histone modifications affect gene expression?

Histone modifications can suppress or activate gene expression.

What is a homozygous dominant individual?

Homozygous dominant individuals would have a genotype of (BB). This means that our example individual would be homozygous for brown eyes.

What does it mean if two genes show a recombination frequency that is anything less than 50%?

If two genes show a recombination frequency that is anything less than 50%, the genes are linked. This is due to the fact that the random assortment of unlinked genes results in 50% recombinant progeny. Hence, the lower the recombination frequency below 50%, the closer the genes are linked.

How does a CVS (chorion villus sampling) help test to see if a fetus has any genetic abnormalities?

In a CVS (chorian villus sampling), a sample of the placenta is physically removed. Chorionic tissue is rich in cells, hence karyotypes can be done in a timely fashion. This method has a higher rish than amniocentesis.

In addition to Mendel's Law of Independent Assortment, there is another measure for nature to create genetic diversity, what is this?

In addition to Mendel's Law of Independent Assortment, there is another measure for nature to create genetic diversity — crossing over of homologous chromosomes.

There are 7 different types of patterns of inheritance, what are they?

In many cases, genetic inheritance is not as simple as having either a dominant or a recessive trait. There could be many others patterns of inheritance cause by different factors. The Charts Summarizes them all:

What happens if there is an inactivation of both X chromosomes.

Inactivation of both X chromosomes would not allow female embryos to develop.

Incomplete dominance is one type of pattern of inheritance, explain this.

Incomplete dominance appears as an intermediate between two phenotypes . A classic example is a white flower mixed with a red flower, producing a pink flower. This shows how the color is Intermediate between white and red). Mnemonic: INcomplete dominance is the INtermediate

What is incomplete dominance (punnet square)?

Incomplete dominance occurs when neither of the two alleles exerts dominance...an intermediate of "blended" phenotype is made. If red flowerd snpadragons were crossed with white-flowered snapdragons, we get pink snapdragons.

what does incomplete penetrance mean in heredity?

Incomplete penetrance is when the trait is expressed in only part of the population that has the allele. This is like saying a person could have blue eyes, even though they have a dominant brown eye allele. In this case, the brown allele is incompletely penetrant because the trait will not be expressed in the full population.

Klinefelter's syndrome is a type of syndrome caused by aneuploidy, explain this syndrome.

Klinefelter's syndrome is a sex chromosome trisomy in males. Specifically, a male with Klinefelter's will have XXY sex chromosomes. This means that they will have 2n+1 (47) chromosomes. This is associated with an intellectual developmental disorder. It is also associated with some physical abnormalities, as well as sterility. This syndrome leaves a man sterile with feminization.

Law of Segregation in one sentence:

Law of Segregation: When gametes (egg or sperm) are made, the two traits carried by each parent separate.

Gregor Mendel is the father of genetics because he used pea plant experiments to learn more about heredity. These experiments led to the creation of Mendel's three laws: 1. Law of Dominance 2. Law of Segregation 3. Law of Independent Assortment.

Law of dominance: this is the law that says dominant alleles will mask the expression of recessive alleles. Recall that we mentioned this earlier - you have Mendel to thank for that! One of the traits Mendel studied was the inheritance of plant height, where the dominant trait was tall (T) and the recessive trait was short (t). Mendel did not automatically know which trait was dominant and which was recessive. He figured it out by counting the number of tall and short offspring produced after a tall and a short plant were crossed.

Gregor Mendel is the father of genetics because he used pea plant experiments to learn more about heredity. These experiments led to the creation of Mendel's three laws. 1. Law of Dominance 2. Law of Segregation 3. Law of Independent Assortment.

Law of segregation: this law states that homologous gene copies will separate during the formation of gametes (meiosis). For example, if an individual carries the trait Aa, then their gametes will carry either allele A or a, but not both at the same time. In the picture below, we can see that the haploid gametes only carry one strand of the chromosome, not two.

What are non-recombinant gametes?

Non-recombinant gametes are parental gametes. Here, two genes are linked on the same chromosome and recombination during prophase I does not separate them. The closer two genes are on a chromosome, the lower the likelihood that they will be separated during prophase I.

Proto-oncogenes are a type of gene defect, explain this.

Proto-oncogenes are genes involved with cell division and regulation (keeping the cell under control). Though the name may sound scary, a proto-oncogene is an important, normal gene involved in cell control. However, if there is a gain-of-function mutation, this can cause the proto-oncogene to become an oncogene.

What are recombinant gametes?

Recombinant gametes are the result of crossing over. This is when crossing over during prophase I has separated two genes that were previously linked on a chromosome. The further apart two genes are on a chromosome, the greater the likelihood that a recombination event will separate them.

Mendel's Law of Independent Assortment in short:

States that genes located on different chromosomes assort independently of each other. In other words, during the forming of gametes the alleles for height segregate independently from the alleles for a trait such as color. The genes for height and color are on different chromosomes and will sort independently. The clother the genes are located on a chromosome, the more likely they will be linked. Genes on the same chromosome are said to be linked genes.

What would occur if further apart two genes are on a chromosome?

The farther apart the two genes are on a chromosome, the greater the frequency of crossover and recombination between them. The result of a crossover is what we call a recombination.

What do you call the result of a crossover?

The result of a crossover is what we call a recombination.

What are your three most important one-gene cross ratios? Read Notion notes for detail explanation.

To save time on the DAT, you need to know your one-gene cross ratios to quickly solve genetics questions using probabilities. Commit these three single allele crosses to memory: Homozygous x homozygous = 1/1 AA or 1/1 Aa or 1/1 aa Homozygous x heterozygous = ½ AA (or aa) and ½ Aa Heterozygous x heterozygous = ¼ AA, ½ Aa, ¼ aa (Remember, this is our 1:2:1 ratio from a monohybrid cross seen above)

What are tumor supressor genes and how are they haplosufficient?

Tumor suppressor genes are also involved with cell regulation, and they help to suppress the chances of developing a tumor. One tumor suppressor gene is sufficient to produce a tumor suppressor effect, and that means tumor suppressor genes are haplosufficient.

What are gene defects and what are some important examples?

We have covered patterns of genetic inheritance under normal circumstances. However, there are also abnormal conditions that can occur. Some example of gene effects are haploinsufficiency, haplosufficiency, proto-oncogenes, gain-of-function mutations, oncogenes, tumor suppressor genes (p53, p21, Retinoblastoma gene (RB), loss-of-function mutations, null alleles,

Example of Law of Dominance with Punnett Square:

When AA is crossed with aa, the offspring is Aa..expressing only the dominant traits. Done as a Punnett-Square:

What is a cross in heredity?

When we talk about heredity and inheritance, we will often mention crosses. As we have seen before, Mendel proposed the three laws of heredity after performing crosses in pea plants. A cross occurs when an two organisms with unique traits are mated to produce offspring.

What is X-activation?

X-activation is a phenomenon that they happen in females. IT basically means that one of a female's X chromosomes becomes inactivated and forms a barr body. This prevents females from producing X chromosome gene products in excess.

How do you find out the number of gametes if you are given a genotype?

You use the 2^n rule where n= number of heterozygotes. For example, how many gametes canbe made given the following genotype: Xx, Yy, Zz, WW, and BB. 2^3 = 8. Why? Because there are three heterozygotes, hence n = 3.

How does a amniocentesis help test a fetus to see if they have any genetic abnormalities?

Amniotic fluid of the fetus is sampled. Fetal secretions and epidermal cells from the Respiratory and GI tract can be cultured and subjected for karyotypic analysis.

What is an allele?

An allele is one variation of a gene that has different forms. For example, there are various color alleles at the eye color gene locus, such as brown eye alleles, blue eye alleles, etc. An allele is a slightly different molecular form of a gene. Often one allele of a pair is dominant and the other is recessive. The allele that is dominant will mask the effect on a trait of its recessive partner. Let A = an allele Let AA = homozygous dominant Let Aa = Heterozygous Lets aa = homozygous recessive

Pleiotropy is a type of pattern of inheritance, explain this.

Another case where genes can affect each other is pleiotropy. Pleiotropy is when one gene affects many different traits. An example of this may be illustrated by cystic fibrosis. Here, a single gene will lead to the expression of cystic fibrosis, a disease that causes the expression of different symptoms in different tissues. Pleiotropy refers to a single gene that can affect an organism in various ways. In sickle cell anemia, a single mutation occurs, and gives rise to a defective hemoglobin molecule. This one gene mutation causes a wide range of problems such a oxygen utilization, and tissue and organ damage. in PKU disease, we see an unusual amount of phenylalanine in the blood. This is due to a mutant gene. Untreated patients have lowers IQs, larger heads, and lighter hair color all due to a single gene!

Codominance is one type of pattern of inheritance, explain this.

Another type of pattern is codominance. In codominance, the contributions of both alleles will be seen. To help you conceptualize this, consider the classic example of ABO blood typing. Here, an allele for A blood and an allele for B blood will produce a phenotype of AB blood. This shows that no blood type is dominant to the other, so they are codominant. Mnemonic: COdominance is NO dominance Another type of intermediate inheritance is called codominance. In codominance, a pair of nonidentical alleles specify two phenotypes...both expressed at the same time in heterozygote. Here is an example: If a person as AB blood type: One parents gives iA allele, other parents give iB allele. We get iAiB which is blood type AB. The A and B alleles are said to be codominant.

What does it mean when a organism is homozygous?

Homozygous individuals have the same allele for a given gene loci of their homologous chromosomes. Continuing with our example of eye color, a homozygote would have a genotype of BB or bb. Both the maternal and paternal chromsomes would have the same allele.

What is a homozygous recessive individual?

Homozygous recessive individuals would have a genotype of (bb), which means they would be homozygous for blue eyes. In this case, there is no dominant allele covering the effect of the recessive allele. This is the only case where a recessive trait would be displayed phenotypically.

How does an ultrasound help test a fetus if they have any genetic abnormalities?

In a ultrasound, a fetus is visualized to see any morphological defects prenatally.

What is a hemizygous individual?

In addition to homozygous and heterozygous, there is another term to genetically describe an individual. Hemizygous individuals will have only one copy of an allele, instead of two. A common example of this occurs in men, because of the special characteristics of their sex chromosomes. Men have one X and one Y chromosome. Contrary to the rest of the chromosomes that come in homologous pairs, X and Y chromosomes are not homologous. Therefore, they do not contain the same gene loci. For this reason, men will have one copy of the alleles found on their X and Y chromosomes.

What is a dihybrid cross?

A dihybrid cross is a little bit more complicated than monohybrid crosses. It examines the pattern of inheritance for two (di = two) genes when heterozygous individuals are crossed. In order for this type of cross to work, the two genes need to be on separate chromosomes. See the diagram below for an example of a dihybrid cross. After we do the Punnett Square, we can see that for the F2 generation, there are a lot more combinations of possible genotypes compared to the previous monohybrid cross. For this reason, we don't need to memorize the genotype ratios since we can always draw the Punnett Square out if we need to. However, to speed things up on the DAT, it might be helpful to remember that the phenotype ratio (dominant : recessive traits) should always be (9:3:3:1). 9 is the phenotype with both dominant traits. (i.e., yellow and round peas) 3 is the phenotype with one recessive and one dominant traits. (i.e., green and round peas) 3 is the phenotype with also one recessive and one dominant traits, just the other way round. (i.e., wrinkled and yellow peas) 1 is the phenotype with both recessive traits. (i.e., green and wrinkled peas).

What is a genotype?

A genotype is the genetic composition of an organism. In other words, it is the actual nucleotide (DNA) sequence for a given allele. For example, the genotype for a green pea plant could be yy. A genotype is the genes making you up.

What is a halotype?

A halotype is a group of genes that are located so close to each other on a chromosome, that they tend to be inherited together. Because the genes are clustered together, they tend to not be split via crossing over, and are often found in offspring.

What can a linkage map be based on?

A linkage map can be made based on the % of cross overs!! Linkage map of 3 genes shown in diagram. The probability of a crossover is proportional to the distance separating the genes!!

What is epigenetics?

A method of genetic regulation that does not touch the DNA code, but instead regulate gene expression by working "on top" of the codes is called epigenetics. Epigenetics is a cellular trait that is heritable to daughter cells, despite the fact that it does not involve the DNA code. Mechanisms that affect a person's epigenetics can be obtained during development, from chemicals or drugs, diet, and aging.

What are pedigree analysis? Read Notion Notes for detail explanation and examples.

A pedigree chart is a tool geneticists use to track how a certain trait has been passed down from generation to generation. Similarly, pedigrees show how traits will pass to future generations. All in all, they allow us to determine how a certain trait is inherited (i.e., autosomal dominant, sex linked, etc.). In a pedigree chart, females are circles, while males are squares. Unaffected individuals are usually unshaded, while affected individuals are shaded.

What is a phenotype?

A phenotype is the observable trait that results from the genotype. For example, the pea plant color phenotype for the genotype yy is green. A phenotype is the physical and physiological traits of an organism..in other words, your observable traits.

What is a wild type allele?

A wild type allele is the normal allele that is the most common in nature. Interestingly, wild type alleles can undergo mutations to form mutant alleles.

Explain Nondisjunction of sister chromatids - mitosis (cell replication):

A. One cell: 2 x (23) + 1 = 2n + 1 (47) B. One cell: 2 x (23) - 1 = 2n - 1 (45) DAT Pro-Tip: Mitosis occurs in somatic cells to make two daughter cells, which normally contain 46 chromosomes (diploid = 2n). Somatic cells can't be fertilized to make a zygote. Note: embryonic nondisjunction is associated with sister chromatids that do not separate during mitotic anaphase. Nondisjunction occurs most often during embryonic development.

Explain Nondisjunction of sister chromatids - during meiosis II:

A. One haploid cell: (23) + 1 = n+1 (24) B. One haploid cell: (23) - 1 = n-1 (22) C. Two haploid cells: (23) - 0 = n (23) A. When the n+1 = 24 gamete fertilizes with a n = 23 normal gamete it produces a zygote with 2n+1 (47) B. When the n-1 = 22 gamete fertilizes with a n = 23 normal gamete it produces a zygote with 2n-1 (45) C. Two of the gametes produced in nondisjunction during meiosis II are n = 23. When either of the n = 23 gamete fertilizes with a n = 23 normal gamete it produces a zygote with 2n (46) (normal)

Explain Nondisjunction of homologous chromosomes - during meiosis I

A. Two haploid cells: (23) + 1 = n+1 (24) B. Two haploid cells: (23) - 1 = n-1 (22) A. When either of the two n+1 = 24 gametes is fertilized with a n = 23 normal gamete, it produces zygotes with 2n + 1 (47) B. When either of the two n-1 = 22 gametes is fertilized with a n = 23 normal gamete, it produces a zygote with 2n - 1 (45) DAT Pro-Tip: Meiosis occurs in gametes to make four daughter cells, which normally contain 23 chromosomes (haploid = n). Fertilization with a gamete from the opposite sex produces a zygote.

Epistasis is a type of pattern of inheritance, explain this.

In addition, different genes can also affect each other's expression. Epistasis is an interaction between genes, where one gene can affect the expression of a different gene. For example, say there are three varieties of human hair: blonde, red and bald. In this example, blonde and red hair colors are controlled by one gene, where blonde (R) is dominant to red (r). The bald (b) gene is an entirely different gene, so it is not a hair color allele. In other words, the haircolor (R vs r) does not matter in a person that is bald (bb), because the bald gene affects the expression of the hair color gene. An epistatic gene is a gene that covers up the expresion of another gene in the phenotype. This epistasis is similar to ordinary dominance except that 2 different genes are involved. In other words, two alleles of a gene mask the expression of another gene's alleles!! In Labrador Retrievers, we see many different colored dogs. This color difference is the result of variations in the melanin pigment. Melanin is made by a variety of enzymes which are made by gene pairs. Alleles of a different gene control the extent to which % of melanin is deposited into the hair. The interactions between the gene pairs produce dogs of different colors. If no melanin is made, an albino will result.

Gregor Mendel is the father of genetics because he used pea plant experiments to learn more about heredity. These experiments led to the creation of Mendel's three laws. 1. Law of Dominance 2. Law of Segregation 3. Law of Independent Assortment.

Law of independent assortment: this states that in meiosis, homologous chromosomes will line up on the metaphase plate independently of one another, so that the separation of alleles is totally random. For example, the maternal copy of chromosome 1 could separate to the left, and the paternal copy is separated to the right. This separation pattern is random based on how homologous chromosomes line up on the metaphase plate, and does not apply to any of the rest chromosomes i.e., the remaining chromosomes have a completely random separation pattern. This contributes to genetic variability during meiosis. During metaphase 1, homologous chromosomes line up in double file, along the metaphase plate (midline). The homologous chromosomes (parental and maternal) will then assort independently. Double file is important to meiosis I. Here, the spindle fibers will attach to either side of a pair of homologs. The two chromosomes will be pulled apart, one homolog to either side. Contrast this with meiosis II where the chromosomes line up in single file. Here, the spindle fibers will attach to either side of the same chromosome, and the two chromatids will be pulled apart, one chromatid to either side. Under the law of independent assortment, if we consider a 6 chromosome diploid organism (haploid number is 3), the 6 chromosome could assort with: look at diagram Look at how the independent assortment of homologous chromosomes leads to genetically different daughter cells. In the context of the human body, each dividing gamete will have a random distribution of how the maternal and paternal versions of 23 homologous chromosome pairs split. That can create 2^23 options, which is a lot of genetic diversity.

What does linkage refer to?

Linkage refers to the tendency of genes which are located on the same chromosome to stay together. Linkage could be disrupted when crossing over occurs in meiosis.

What are linked genes?

Linked genes are those that are found close together on the same chromosome. How closely those genes are linked will determine how rarely they are separated by a crossover event. From this, scientists can deduce the relative distances between linked genes just by looking at the recombination frequencies.

What do loss-of-function mutations do and how do they affect tumor suppressor genes?

Loss-of-function mutations cause a loss in a gene-product's function (the protein doesn't do its job properly). Tumor supressor genes are normally always "on" to suppress tumors. A loss of function mutation creates a null allele. A loss-of-function mutation renders a tumor suppressor gene inactive, and a loss of function on both tumor suppressor gene copies means that the cell will lose that specific tumor supressing function because tumor supressor genes follow a two hit hypothesis. Tumor suppressor genes → two loss of function mutations → cancerous null alleles

What are map units (aka m.u. also called centimorgans)?

Map units (aka m.u. also called centimorgans) are units that geneticists use to infer the distance between genes on a chromosome. One map unit is defined as the chromosomal distance that would allow 0.01 crossover events per generation. For example, if two genes are 20 m.u. apart, 20*0.01*100% = 20% of the time recombination will occur between the genes.

What is nondisjunction?

Nondisjunction is the failure of one or more chromosome pairs to separate properly during anaphase. This results in the production of daughter cells with the incorrect number of chromosomes. Nondisjunction can occur in three forms: 1. Nondisjunction of homologous chromosomes - during meiosis I 2. Nondisjunction of sister chromatids - during meiosis II 3. Nondisjunction of sister chromatids - mitosis (cell replication)

Null alleles are a type of gene defect, what are they?

Null alleles have mutations that create in inactive, or 'useless' version of a gene that lacks its normal function. For example, a null allele in a person's melanin gene loci (at both homologs) would result in no production of functional melanin pigment. Tumor suppressor genes → two loss of function mutations → cancerous null alleles

Oncogenes is a type of gene defect, explain this.

Oncogenes either make too much protein, or they make a protein product that is too active. For this reason, oncogenes are produced via a gain in function mutation, which causes the cell to become cancerous. Proto-oncogene → one gain in function mutation → cancerous oncogene

What does it mean when an organism is heterozygous?

Organism's are said to be heterozygous when they possess a dominant allele on one of their homologous chromosomes (homologs), and a recessive allele (for the same gene) on the other homolog. In our example from above, the person would have a genotype of (Bb). For this reason, they would be considered a heterozygote. Remember, we said brown was going to be the dominant allele, so they will display the brown eye phenotype.

What is penetrance in heredity?

Penetrance refers to the proportion of individuals who exhibit the phenotype of an allele for a given gene. These proportions can vary, which is why some alleles are said to be completely penetrant, while others are said to be incompletely penetrant.

Polygenic inheritance is a type of pattern of inheritance, explain this.

Polygenic inheritance is when multiple genes affect one trait with continuous variation. For example, consider a person's height. This varies tremendously from person to person. As such, it is a single phenotype that is influenced by separate genes. Here we see two or more genes contributing to a single trait. Hair color, skin color, and height result from the interactions of several genes. Height is a good example. Unlink pea plants, we don't appear as "short" or "tall"...but many different heights are seen. When several genes are involved, a bell-shaped curve is often used to describe the phenotype.

Proto-oncogenes follow a one hit hypothesis. What does that mean?

Proto-oncogenes follow a one hit hypothesis, meaning that a gain of function mutation in just one of their genes is enough to produce an oncogene that causes cancer.

What are punnett squares?

Punnett squares are tools that are often used to help in the visualization of crosses. Punnett squares are helpful if you are solving genetics questions about single locus crosses. However, they quickly become complex and confusing if you need to solve a question about multi-locus crosses, as was shown above for the dihybrid cross.

Retinoblastoma gene (RB) is a type of important tumor suppressor genes that can be found. Explain this type of tumor suppressor gene. Keep in mind that loss of function mutations in both copies of these genes are required for cancer-causing, null tumor suppressor alleles.

Retinoblastoma gene (RB) gives rise to retinoblastoma protein (pRb) that prevents excessive cell growth during interphase of the cell cycle. When the cell is ready to divide, pRb will be phosphorylated so the cell can grow appropriately.

Pleiotropy and polygenic inheritance are essentially opposites, explain how.

So, we can see now that pleiotropy and polygenic inheritance are essentially opposites. Pleiotropy = one gene gives many different traits Polygenic inheritance = one trait given by many different genes. DAT Pro-Tip: polygenic inheritance = poly genes = multiple genes as compared to pleiotropy which does not contain the suffix 'poly' within it. The image illustrates both pleiotropy and polygenic inheritance. You can see here one gene influence multiple traits (i.e., gene 1 influences phenotype 1 and phenotype 2). We also see polygenic inheritance (i.e., phenotype 2 is influenced by genes 1, and 2, and 3).

What is aneuploidy?

So, we can see that nondisjunction leads to an abnormal number of chromosomes in the resulting daughter cells; this is referred to as an aneuploidy. If an aneuploid gamete is fertilized, a number of syndromes may result. So, when the offspring have the wrong number of chromosomes, this is called aneuploidy. Some of these syndromes are characterized by having an extra copy of a chromosome, which is referred to as trisomy (tri = 3, normally we only have 2 copies). Similarly, some of the syndromes are called monosomy (mono = 1) because they are missing a copy of a chromosome.

Multiple alleles is a type of pattern of inheritance, explain this.

Some genes are said to have multiple alleles. These genes will have more allele options than a gene with a simple, dominant or recessive, two allele option. Our example of ABO blood typing from above also shows how genes can have multiple alleles. This is because there are more than two options for blood type, as a person can be type A, AB, B, or O. Multiple alleles is when a gene has more than two given alleles. Let us consider an example usisng the three allele gene..the ABO blood type in humans. There could be four possible phenotypes A, B, O, AB...all produced from combinations of three different alleles: iA, iB, and iO. Types O and AB are produced from a single genotype, but A and B may be either homozygous or heterozygous.

What is the F1 generation, (filial 1 hybrid)?

The F1 generation is produced after true breeding parents are crossed. The F1 generation is the first generation of offspring, and it may also be referred to as the filial 1 hybrid. As we have deduced, a cross between two true breeding parents will produce offspring that are all heterozygous.

What is the F2 generation, (Filial 2 hybrid)?

The F2 generation will be produced after the heterozygous F1 offspring are crossed. This generation is known as the filial 2 hybrid, and it is the second generation of offspring that are produced in a monohybrid cross. The F2 generation would have some that are homozygous and some that are heterozygous for the trait. This is the generation that Mendel derived his three laws from.

What is dominant or recessive mean?

The alleles that make-up genotypes and phenotypes are said to be dominant or recessive. Dominant alleles will mask the expression of recessive alleles. This is important because humans have pairs of chromosomes called homologous chromosomes. In the pea example above, the yellow allele (Y) is dominant to the recessive allele (y). Therefore, when you have a plant with genotype Yy, the dominant yellow Y allele creates the phenotype of yellow. Only with two recessive green alleles (yy) and the absence of a dominant allele, will the recessive phenotype of green be seen.

What is the first stage of creating genetic diversity?

The first stage of creating genetic diversity is during prophase of meiosis I, when homologous chromosomes physically cross over and join together to form a tetrad or bivalent. The location where the chromosomes actually cross is known as a chiasma. This is where maternal and paternal genetic material is exchanged. After homologs cross over during prophase I, there will be genetically unique chromatids. As a whole, these chromatids have a different genetic makeup than either of the original parental chromosomes.

What is a genome?

The genome is all of the DNA within a cell.

What are wild type alleles?

The wild type allele is the normal allele that is the most common in nature. Interestingly, wild type alleles can undergo mutations to form mutant alleles.

How can the three histone modifications, DNA methylation, DNA de-acetylation, and DNA acetylation be inherited?

These histone modifications can be epigenetically inherited. Epigenetic modifications can be inherited, even though it does not involve the DNA code itself.

How can traits be accumulated in later generations and how can this lead to evolutionary changes?

Traits can accumulate in later generations because offspring acquire their genetic information from parents. This can lead to evolutionary changes by natural selection, if the accumulated traits increase fitness?

What are the two ways traits can be passed?

Traits can be passed sexually, as is the case for most animals; or, they can be passed asexually, as would be the case in a bacterium undergoing binary fission.

How is genomic imprinting similar and different from sex-linked traits?

Traits that are genomically imprinted are similar to sex-linked traits because their expression is regulated by whether they are maternal or paternal. However, an important difference from sex-linked traits is that genomic imprinting can occur on autosomal chromosomes as well.

Tumor supressor genes follow a two hit hypothesis, what does that mean?

Tumor suppressor genes follow a two hit hypothesis. This means that loss of function mutations are required in both of their genes in order to be cancer-causing. This is because tumor suppressor genes are haplosufficient (one gene copy is enough to maintain normal function).

Turner syndrome is a type of syndrome caused by aneuploidy, explain this syndrome.

Turner syndrome is an X chromosome monosomy that affects females. This means that a female will have one X chromosome, leading to 2n-1 (45) chromosomes. Usually, it does not cause any intellectual disability; however, it does lead to physical abnormalities and sterility. A female will have no barr bodies and hence been sterile with poorly developed ovaries and incompletely developed secondary sex characteristics. O

Which blood type is a universal donor and universal recipient? What could result is a heterozygous man for Type A blood and a heterozygous woman for Type B blood has a child? Explain each blood type and the type of surface antigens and antibodies they would each have.

Type O - Universal Donor Type AB - Universal Recipient What could result if a heterozygous man for Type A blood and a heterozygous woman for Type B blood has a child? Set up a punnett Square!! Type A blood has "A" surface antigens..but no B Type B blood has "B" surface antigens...but no A Type AB blood has both A and B surface antigens Type O has neither A nor B surface antigen Type A blood, for example, has "A" surface antigens, thus obviously no anti-A antibodies will be made since it would destroy them. If Type B blood was injected into a Type A person, anti-B antibodies in their plasma will recognize the "foreign invader" and clumping or agglutination would occur to cleanse the blood to foreign protein. Type O do not produce antigens, thus, their blood is not normally rejected. Those with Type AB blood do not many any antibodies..this blood type does not discriminate, thus can be universal "recipient" in transfusions. Summary: Type A: A antigens on surface; B antibodies in plasma Type B: B antigens on surface; A antibodies in plasma Type AB: A and B antigens on surface, No A and No B antibodies in plasma Type O: No A and No B surface antigen; B and A antibodies in plasma

p21 is a type of important tumor suppressor genes that can be found. Explain this type of tumor suppressor gene. Keep in mind that loss of function mutations in both copies of these genes are required for cancer-causing, null tumor suppressor alleles.

p21 is another tumor suppressor gene. It inhibits cyclin dependent kinase (CDK) activity, which stimulates cell division. Therefore, p21 decreases the frequency of cell division. Remember, cancer cells are cells that usually divide and grow out of control.

p53 is a type of important tumor suppressor genes that can be found. Explain this type of tumor suppressor gene. Keep in mind that loss of function mutations in both copies of these genes are required for cancer-causing, null tumor suppressor alleles.

p53 is the most well-known and well studied tumor suppressor gene, and it is considered the guardian of the cell. It has numerous roles in cancer prevention, and it is upregulated (increases in its protein production) in response to cell stress.