Biology Unit 5: Heredity

The rule of addition

(one of the two basic laws of probability that you will use directly in solving genetics problems to help understand how to predict offspring of genetic crosses) When calculating the probability that any of two or more mutually exclusive events will occur, you need to add to gather their individual probabilities. For example, if you are tossing a die, what is the probability that it will land on either the side with four spots or the side with five spots ? ( % + y = 1/3 )

The rule of multiplication

(one of the two basic laws of probability that you will use directly in solving genetics problems to help understand how to predict offspring of genetic crosses) When calculating the probability that two or more independent events will occur together in a specific combination, multiply the probabilities of each of the two events. Thus, the probability of a coin landing face up two times in two flips is / 2 x = 1/4.

locus

(plural , loci ) the location of a gene on a chromosome.

fertilization—

(the combination of a sperm cell and an egg cell), one haploid gamete from the father fuses with one haploid gamete from the mother. The result is a fertilized egg called a zygote. It is diploid ( has two sets of chromosomes ) and may be symbolized by 2n.

Prophase II

A spindle apparatus forms , and sister chromatids move toward the metaphase plate

sex chromosomes

An exception to the rule that all chromosomes are part of a homologous pair may be found with these. in humans , it is the X and Y. Hy man females have a homologous pair of chromosomes, XX , but males have one X chromosome and one Y chromosome.

Clone

An individual that reproduces asexually gives rise to this, a group of genetically identical individuals.

Metaphase 1

At this point in meiosis the homologous pairs of chromosomes are lined up at the metaphase plate. There are four chromosomes and two homologous pairs found in the cell.

Random fertilization

Because each egg and sperm is different, as a result of independent assortment and crossing over, each combination of egg and sperm is unique. In humans, there are about 70 trillion (2^23 x 2^23) possible combinations — and this is all without considering crossing over!

who genes present in mitochondria and plastids are inherited from

Genes that are present in mitochondria and plastids are inherited only from the mother because the zygote's cytoplasm comes only from the egg. You inherited your mitochondrial DNA only from your mother, your mother inherited her mitochondrial DNA only from her mother. Your mitochondrial DNA is your maternal grandmother's! The same maternal inheritance pattern holds for both mitochondria and chloroplasts in plants

Fetal (cells, DNA, testing)

Genetic testing of a fetus may detect certain genetic disorders. Two common tests are amniocentesis and chorionic villus sampling (CVS). Fetal cells and fetal DNA can be isolated from its mother's blood and analyzed . This relatively new noninvasive technique is increasingly being used in fetal screening.

Anaphase I

Homologous pairs separate and move toward the poles spindle apparatus helps move the chromosomes toward opposite ends of the cell. Sister chromatids stay connected.

Independent assortment of chromosomes

In metaphase I, when the homologous chromosomes are lined up on the metaphase plate , they can pair up in any combination with any of the homologous pairs facing either pole. This means that there is a 50 % chance that a particular daughter cell will get a maternal chromosome or a paternal chromosome from each of the homologous pairs. In humans, there are about 8.4 million ( 223 ) possible chromosome combinations resulting from independent assortment!

Environmental factors that also influence gene expression

Many genes or groups of genes display phenotypic plasticity, which occurs when individuals of a population have the same genotype but different phenotypes in different environments. For example, the effect of increased sunlight on melanin production can yield identical twins with very different skin tones

The law of independent assortment

Mendel's second law . It states that each pair of alleles will segregate ( separate ) independently during gamete formation . In other words , chromosomes 1 , 2 , and 4 from the mother and chromosome 3 from the father may go to one daughter cell , while chromosome 3 from the mother and chromosomes 1 , 2 , and 4 from the father go to the other daughter cell . The maternal and paternal chromosomes sort independently . This occurs during anaphase I of meiosis

autosomes

Nonsex chromosomes -- that is all the chromosomes except the X and Y. Haploid cells— are Gametes, meaning sperm and ova ( eggs ) contain half the number of chromosomes of somatic cells , one chromosome of each homologous pair . In humans , gametes contain 22 auto somes plus a single sex chromosome ( X in female , either X or Y in male ) , giving them a haploid number of 23. The haploid number of chromosomes is symbolized by n .

Interphase

Replication occurs , creating a copy of each chromosome . The replicated chromosome has two identical sister chromatids . This roughly doubles the amount of DNA in the cell

Anaphase II

The centromeres of the sister chromatids separate, and individual chromosomes move to opposite ends of the cell

Telophase II and cytokinesis

The chromosomes have moved all the way to opposite ends of the cell, nuclei reappear, and cytokinesis occurs. Each of the four daughter cells has the haploid number of chromosomes and is genetically different from the other daughter cells and from the parent cell

Meiosis I

The first cellular division in meiosis is referred to as meiosis I. Meiosis I begins with a diploid cell.

Metaphase II

The haploid number of chromosomes is now arrayed on the metaphase plate. Because of crossing over, the sister chromatids are not genetically identical. The kinetochores of each sister chromatid are attached to microtubules from opposite poles

Telophase I and cytokinesis

The homologous chromosomes move until they reach the opposite poles and cytokinesis ( the division of the cytoplasm ) occurs. -Each daughter cell contains a haploid set of chromosomes, with each chromosome still consisting of two sister chromatids. -Although the sister chromatids are still attached to each other, the homologous pairs have separated. The chromosome number has been cut in half. Note that the daughter cells are now haploid as they now have only one chromosome of each homologous pair. -At metaphase I, independent assortment occurs.

P (parental) generation

True-breeding parents in a genetic cross F1 (first filial) generation— offspring of the p generation.

Prophase I

Understanding prophase I is critical to understanding meiosis . Study the unique events of prophase I carefully. •The chromosomes condense Sister chromatids are attached at their centromeres. •Synapsis occurs. This perfect alignment is necessary for the next step crossing over. •Crossing over occurs. •where crossing over has occurred (two to three times per homologous pair) chiasmata form, which hold the homologs together until anaphase I. •After crossing over, the spindle poles move away from each other, the nuclear envelope disintegrates, and the spindle microtubules attach the kinetochores forming on the chromosomes. The microtubules then begin to move the chromosomes to the metaphase plate of the cell

monohybrid cross

a cross intended to study only one character (for example, flower color), whereas a dihybrid cross is a cross intended to study two characters (for example, flower color and seed shape). Figure 5.4 shows the results of a dihybrid cross. In this case in the parental generation, two homozygous plants are crossed: one homozygous dominant for yellow and round seeds (YYRR) and one homozygous recessive for green and wrinkled seeds (vyrr). The only gamete the first parent can produce is YR, and the only gamete the second parent can produce is yr. The F1 generation, therefore, is composed of all heterozygous individuals with genotype YyRr . Crossing YvRr with a second YvRr gives an F2, generation. Figure 5.4 shows the possible predicted offspring with their genotypes and phenotypic ratios

pedigree

a diagram that shows the relationship between parents and offspring across two or more generations. In a typical pedigree circles represent females, and squares represent males, White open circles or squares indicate that the individual did not or does not express a particular trait, whereas the shaded ones indicate that the individual expresses or a pressed that trait. Through the patterns they reveal, pedigrees can help deter of individuals that comprise them, pedigrees can also help the genome predict the genome of future offspring.

zygote

a diploid that results from fertilization and may be symbolized by 2n.

Achondroplasia

a form of dwarfism that results from a relatively rare dominant allele. Some dominant alleles cause lethal diseases, but usually only lethal alleles that act late in life are passed on

A linkage map

a genetic map that is based on the percentage of crossover events

The karyotype of an organism

a picture of its complete set of chromosomes , arranged in pairs of largest pair to the smallest pair

asexual reproduction

a single parent passes copies of all its genes to its offspring . The new offspring arise by mitosis and have virtually exact copies of the parent's genome.

Incomplete dominance

a type of dominance in which the F, hybrids have an appearance that is in between that of the two parents. For example, if two plants, one with white flowers and one with red flowers, were crossed and all of the offspring had pink flowers, you could conclude that the trait for flower color exhibits incomplete dominance. Breeding two of the hybrids with in complete dominance gives a flower ratio of 1 red : 2 pink : 1 white.

Somatic cells

all cells in the body that are not gametes . Each somatic cell in humans has 46 chromosomes. Liver cells and neurons are examples of somatic cells

Duchenne muscular dystrophy

an X - linked disorder characterized by a progressive weakening of the muscles and loss of coordination . Affected individuals rarely live past their early 20s

Hemophilia

an X linked disorder characterized by having blood with an inability to clot normally, caused by the absence of proteins required for blood clotting

Huntington's disease

caused by a lethal dominant allele. It is a degenerative disease of the nervous system that usually doesn't affect an individual until he or she is over 40 years old

Cystic fibrosis

caused by a mutation in an allele that codes for a cell membrane protein that functions in the transport of chloride ions into and out of cells. The resulting high extracellular levels of chloride cause mucus to be thicker and stickier, leading to organ malfunctions and recurrent bacterial infections

Sachs disease

caused by an allele that codes for a dysfunction enzyme, which is unable to break down certain lipids in the brain. As these lipids accumulate in the brain cells, a child with the disease will suffer from blindness, seizures, and degeneration of brain function, leading to death

Sickle-cell disease

caused by an allele that codes for a mutant hemoglobin molecule that forms long rods when the oxygen levels in the blood are low. These long rods cause the red blood cell to sickle, clogging small blood vessels and leading to pain, organ damage, and even paralysis

Turner syndrome

caused by nondisjunction in meiosis. a monosomic condition in which the female has just one sex chromosome, often designated XO. Turner syndrome females are sterile because the reproductive organs do not mature. Turner syndrome is the only known viable monosomy in humans

Klinefelter syndrome

caused by nondisjunction in meiosis. occurs when a male possesses the sex chromosomes XXY (an extra X). Klinefelter males have male sex organs but are sterile

Down syndrome

caused by nondisjunction in meiosis. the result of having an extra chromosome 21 (trisomy 21). Down syndrome includes characteristic facial features, short stature, heart defects, and developmental delays

chiasmata

crisscrossed regions that form where crossing over has occurred (two to three times per homologous pair) and hold the homologs together until anaphase I.

Complete dominance

dominance in which the heterozygote and the homo zygote for the dominant allele are indistinguishable. A Yy yellow seed is just as yellow as a YY yellow seed.

testcross

done to determine if an individual showing a dominant trait is homozygous or heterozygous. A testcross is always done between the unknown genotype and a homozygous recessive individual. If the unknown parent is homozygous dominant (RR X rr), all the offspring will show the dominant trait, but if the unknown parent is heterozygous (Rr x rr), some of the offspring will show the recessive trait

A map unit

equal to a 1 % recombination frequency. Map units are used to express relative distances along the chromosome

four daughter cells

final result of meiosis, each of which has half as many chromosomes as the parent cell - one chromosome from each homologous pair . On the other hand, the final result of mitosis is two identical daughter cells with the same number of chromosomes as the parent cell.

The chromosome theory of inheritance

genes have specific locations (called loci) on chromosomes and that it is chromosomes that segregate and assort independently. It is important to connect this physical movement of chromosomes in meiosis to Mendel's laws of inheritance. Pay particular attention to the explanations of the law of segregation and the law of independent assortment.

A heterozygous organism

has two different alleles for a trait (Rr). Phenotype—refers to an organism's expressed physical traits, and genotype refers to an organism's genetic makeup. For example, the phenotype of a seed might be round, and its genotype could be RR or Rr.

diploid

has two sets of chromosomes

Meiosis and mitosis comparison

have some features in common but very different outcomes . Both are preceded by the replication of the cell's DNA , for instance , but in meiosis this replication is followed by two stages of cell division , meiosis I and meiosis II , while there is only one division in mitosis. meiosis results in four daughter cells, each with half the number of the parent cell's chromosomes, and mitosis results in two identical daughter cells with the same number.

Homozygous organisms

have two of the same alleles for a particular trait. If the dominant allele for a trait is designated as R (dominant traits are generally capitalized) , and the recessive allele is designated r (recessive traits are generally not capitalized) , then an individual could be homozygous for the dominant trait (RR) or homozygous for the recessive trait (rr).

Meiosis II

his is the second cellular division in meiosis. Meiosis II begins with a haploid cell

independent assortment

homologous pairs line up on the metaphase plate and are separated and pulled toward the poles in anaphase I, sorting maternal chromosomes from paternal chromosomes. Because the maternal and paternal chromosome of each pair sort randomly, it is known as this. The result of independent assortment is an increase in genetic variation

Chorionic villus sampling

involves using a narrow tube inserted through the cervix to suction out a tiny sample of the placenta that contains only fetal cells. A karyotype can immediately be developed from these cells.

Linked genes

located on the same chromosome and therefore tend to be inherited together during cell division

multiple alleles

many genes exist in more than two allelic forms, and these are these traits. A good example of this is in human blood types. There are three alleles for human blood types: IA , P , and i. A person can receive any combination of two alleles.

Nondisjunction

occurs when the members of a pair of homologous chromosomes do not separate properly during meiosis I or sister chromatids don't separate properly during meiosis II. As a result of nondisjunction, one gamete receives two copies of the chromosome, whereas the other gamete receives none. If the faulty gametes engage in fertilization, the offspring will have an incorrect chromosome number

Amniocentesis

occurs when the physician removes amniotic fluid from around the fetus. The amniotic fluid can be utilized to detect some genetic disorders, and the cells in the fluid can be cultured for a karyotype

Codominance

occurs when two alleles are dominant and affect the phenotype in two different but equal ways. The traditional example for this type of dominance is human blood types. The alleles for A and B blood are dominant to the allele for type O blood , but A and B are codominant to each other. A person who has alleles for both A and B blood will have blood type AB because these alleles are each completely expressed.

F2 (second filial) generation

offspring of F1 population if it is crossed. Mendel's model explains the 3 : 1 inheritance pattern that he observed among F2 offspring. These are the four related concepts: 1. Alternative versions of genes account for variations in inherited characteristics among offspring . For example , consider flower color in peas . The gene for flower color in pea plants comes in two versions : white and purple . These alternative versions of the gene , called alleles , are the result of slightly different DNA sequences . 2. For each character , every sexually reproducing organism inherits one allele from each parent . 3. If the two alleles are different , then the dominant allele will be expressed in the offspring , whereas the recessive allele will have noticeable effect on the offspring . 4. The two alleles for each character separate during gamete production . If the parent has two of the same alleles , then the offspring will all get that version of the gene , but if the parent has two different alleles for a gene , each offspring has a 50 % chance of getting one of the two alleles . This is Mendel's law of segregation.

recombinants

offspring with phenotypes different from either parent

parental types

offspring with the same phenotype as one of the parents

A sex-linked gene

one located on a sex chromosome (X or Y in humans). exhibit unique patterns of inheritance. In humans, there are two types of sex chromosomes, X and Y. A person who inherits two X chromosomes usually develops as a female, whereas a person who inherits one X and one Y chromosome usually develops as a male. I Sex-linked genes may be either X-linked or Y-linked. Figure 5.6 shows the unique pattern of inheritance in an X-linked recessive trait . In addition to tracking the gene from one generation to the next, it is also necessary to track the sex of the offspring. As you study the previous three examples , notice that each of the following are demonstrated : •Each egg contains an X chromosome; there are two types of sperm: those with an X chromosome and those with a Y chromosome. In fertilization, there is a 50 % chance that a sperm carrying an X or Y chromosome will reach and penetrate the egg first. Thus, in humans, sex is determined by chance and by the male sperm cell. Fathers pass X-linked genes on to their daughters but not to their sons; fathers pass the Y chromosome to all their sons. •Females will express an X-linked trait exactly like any other trait, but males - with only one X chromosome — will express the allele on the X chromosome they inherited from their mother. The terms homozygous and heterozygous do not apply to a male pattern of sex - linked genes. The vast majority of genes on the X chromosome are not related to sex. Several X-linked disorders have medical significance

Homologous chromosomes

pairs of similar (not identical) chromosomes. One homologous chromosome from each pair is inherited from each parent. in other words , half of the set of 46 chromosomes in your somatic cells was inherited from your mother (maternal chromosomes), and the other half was inherited from your father (paternal chromosomes) -Both chromosomes of each pair carry genes that control the same inherited characteristics . If a gene for eye color is found at a specific locus on one chromosome , its homolog will have the same gene at the same locus . -Homologous chromosomes are similar in length and centromere position , and they have the same staining pattern.

Gametes

reproductive cells in animals and plants that transmit genes from one generation to the next

Dominantly inherited disorders

require only one copy of the allele in order for the disorder to be expressed. A common misconception is that dominant alleles are more common than recessive alleles because dominant alleles are always expressed. Both of the disorders just described are examples of traits for which the frequency of the dominant allele is very low.

Recessively inherited disorders

require two copies of the defective gene for the disorder to be expressed

Genes

segments of DNA that code for the basic units of heredity and are transmitted from one generation to the next

Crossing over

the DNA from one homolog is cut and exchanged with an exact portion of DNA from the other homolog. Each chromosome is now a mix of maternal and paternal genes because a small part of the DNA from one parent is exchanged with the DNA from the other parent. The result of crossing over is an increase in genetic variation. During prophase I, the exchange of genetic material on homologous chromosomes between non-sister chromatids occurs. Notice that all four chromatids that make up the tetrad are different because of crossing over. In metaphase II, when sister chromatids separate, each chromatid is unique, thus increasing variation. it explains why some linked genes get separated during meiosis . During meiosis, unlinked genes follow independent assortment because they are located on different chromosomes. Linked genes are located on the same chromosome and would not be predicted to follow independent assortment. However, sometimes genetic crosses give results that seem to indicate that some independent assortment has occurred, even when genes are on the same chromosome. These results are not caused by independent assortment but can be explained by crossing over. Research indicates that the farther apart two genes are on a chromosome, the higher the probability that crossing over will occur between them. The likelihood of crossing over between different genes on the same chromosome is expressed as a percentage chance

Synapsis

the joining of homologous chromosomes along their length. This newly formed structure is called a tetrad and precisely aligns the homologous chromosomes gene by gene

Genetic recombination

the production of offspring with a new combination of genes inherited from the parents. This new combination results from crossing over during prophase 1 of meiosis. Many genetic crosses yield some offspring with the same phenotype as one of the parents (these offspring are referred to as parental types) and some offspring with phenotypes different from either parent (these offspring are referred to as recombinants)

Meiosis

the type of cell division that reduces the numbers of chromosomes. homologous chromosomes from the largest One chromosome of each homologous pair is randomly distributed to gametes. Chromosome number is now reduced by half; each gamete has one chromosome of every homologous pair.

sexual reproduction

two individuals (parents) contribute genes to the offspring. This form of reproduction results in greater genetic variation in the offspring than asexual reproduction.

polygenic inheritance

two or more genes have an additive effect on a single character in the phenotype (such as height or skin color in humans). When several genes are involved, the phenotype usually is described by a bell-shaped curve, with fewer individuals at each extreme and most individuals clustered in the middle.