Unit 8 - Genetics

Aneuploidy result from nondysfunction

A chromosomal aberration in which one or more chromosomes are present in extra copies or are deficient in number.

Genetic disease

A disease is an illness that is caused by a gene, most are caused by recessive alleles of a gene. A person with a dominant allele and a recessive allele do not exhibit symptoms but may pass on the recessive allele which may potentially confer the disease and its symptoms upon offspring if the other parent does the same. Thus, such individuals are carriers of the disease. Cystic fibrosis is an example. Dominant genetic diseases are unlikely. Being a carrier is not possible in such an instance.

Dominant and Recessive alleles

A dominant allele maks the efect of the recessive allele, meaning that altough the reecessive allele is in the genotype, it is not expressed in the phenotype. Rr The general reason for dominance is that one allele which is dominant to another code for a protein which is active and carries out a function. Whereas the recessive allele codes for a non-funcitonal protein.

Bivalent

A pair of homologous chromosomes is bivalent and the pairing process is sometimes called synapsis. A pair of homologous chromosomes contains four chromatids and is sometimes called a tetrad. Five chiasmata are visible in this tetrad (textbook page 162, figure 6), showing that crossing over can occur more than once. In Meiosis as in Mitosis there are 3 stages during interphase G1 S and G2. During S phase the DNA is replicated so each chromosome consists of two chromatids. Then synapsis occurs where homologous chromosomes come to align beside each other. This is referred to as a tetrad or bivalent. In many eukaryotic cells a protein-based structure forms between the homologous chromosomes called the synaptonemal complex.

Y chromosome

A small part of the chromosomes the same sequence of genes as a small part of the x chromosome, but the genes on the remainder of the y chromosome are not found on the x chromosome and are not needed for female development. One y chromosome gene causes a fetus to develop male in particular: the SRY or TDF (Testes determining factor), which initiates the development of male features including testes and testosterone. Thus an X and a Y form a male, while two X chromosomes and no Y lead to the formation of a female (e.g. ovaries instead of testes) and female sex hormones not testosterone due to the lack of the TDF gene.

Somatic cell

A somatic cell, or vegetal cell, is any biological cell forming the body of a multicellular organism other than a gamete, germ cell, gametocyte or undifferentiated stem cell.

Crossing over

After synapsis a process called crossing over occurs. A junction is created where one chromatid in each of the homologous chromosomes breaks and rejoins with the other chromatid. Crossing over occurs at random positions anywhere along the chromosomes. At least one crossover occurs in each bivalent and there can be several. Because a crossover occurs at precisely the same position on the two chromatids involved, there is a mutual exchange of genes between the chromatids. As the chromatids are homologous but not identical, some alleles of the exchanged genes are likely to be different. Chromatids with new combinations of alleles are therefore produced.

Autosome

Any chromosome that is not a sex chromosome

Eukaryotic chromosomes

Are linear DNA molecules associated with histone proteins. Are composed of DNA and protein. The DNA is a single immensely long linear DNA molecule. It is associated with histone proteins.

non-kinetechore fibers

Are responsible for elongating the cell during anaphase and they can also help align and separate chromosomes (They can push the centrosomes to opposite poles of the cell).

monosomy

Chromosomal abnormality consisting of the absence of one chromosome from the normal diploid number

General trend increasing genome size increased complexity If this is always the case is the question

Chromosome number

Difference Eukaryotic Chromosomes

Chromosomes too thin to see with a light microscope during interphase (need supercoiling) Chromosomes differ in length and position of the centromere. At least two different chromosomes in each species for eukaryotes. Every gene in eukaryotes possesses a certain position on one type of chromosome called the locus of the gene. (Having genes arranged in a standard sequence along a type of chromosome allows parts of the chromosomes to be swapped during meiosis)

Plasmids

Commonly found in prokaryotes (unusual in eukaryotes). Small, circular, and naked. Contain a few genes which may be useful to the cell but not for the basic life processes. Are essential to the survival of a bacteria cell in a certain environment (like if have to react by changing protein, etc.). Can share plasmid and replicate independently of regular cell replication cycle if want. Plasmids can be shared across species (when gain a resistance can share that resistance in the form of plasmids). Sharing across species occurs when an organism dies and releases its plasmid, another species may absorb it. Thus, plasmids are also used by biologists to transfer genes between species artificially. As plasmid genes do not always replicate at the same rate as the cell cycle, there may be multiple different plasmids in a prokaryote, and plasmids may not always be passed on. There may also be multiple copies of the same plasmid in a prokaryote. E.g. antibiotic resistance genes may be present in plasmids.

Centromere

Connects the p and q arms (p top and q bottom)

Allele

Differences in certain characteristics of an organism, e.g. a tall pea plant vs a short one, are due to differences in heritable factors. Specifically, these differences in the heritable factor of the height of a pea plant are due to alternative forms of the same gene. These different forms of the same gene are called alleles. There can be more than two alleles for a gene. E.g. there are three alleles in humans that determine one's ABO blood group. As alleles are different forms of the same gene they possess the same locus. Only one allele can occupy the locus on a chromosome. Most animal and plant cells have two copies of each type of chromosome. Therefore, there are generally two copies of each gene present. These could be two copies of the same allele of a gene or two different alleles. Different alleles have slight variations in their base sequence. Usually only one or a very small number of bases are different.

Types of Variation

Discrete - blood types Continuous - Skin color

Test cross

Don't know the genotype of a dominant phenotype, can perform a test cross. A dominant phenotype that could be either homozygous dominant or heterozygous, cross it with a known homozygous recessive.

Exchange of genetic material

During prophase 1 of mitosis breaks occur in the DNA, non-sister chromatids "invade"/attach to the breaks on a homologous chromosomes (on a non-sister chromatid). When crossing over is complete the non-sister chromatids continue to adhere to the site where crossing over occurred. These connection points are called chiasmata (plural) or chiasma (singular). Evidence suggests that chiasmata connections are essential for the successful completion of meiosis.

Meiosis outline

During the early stages of meiosis the chromosomes gradually shorten by supercoiling. As soon as they become visible it is clear that each chromosome consists of two chromatids. This is because all DNA in the nucleus is replicated during the interphase before meiosis, so each chromosome consists of two sister chromatids. Initially the two chromatids that make up each chromosome are genetically identical. This is because DNA replication is very accurate and the number of mistakes in the copying of the DNA is extremely small. We might expect the DNA to be replicated again between the first and the second division of meiosis, but it does not happen. This explains how the chromosome number is halved during meiosis. One diploid nucleus, in which each chromosome consists of two chromatids, divides twiceto produce four haploid nuclei in which each chromosome consists of one chromatid.During the early stages of meiosis the chromosomes gradually shorten by supercoiling. As soon as they become visible it is clear that each chromosome consists of two chromatids. This is because all DNA in the nucleus is replicated during the interphase before meiosis, so each chromosome consists of two sister chromatids. Initially the two chromatids that make up each chromosome are genetically identical. This is because DNA replication is very accurate and the number of mistakes in the copying of the DNA is extremely small. We might expect the DNA to be replicated again between the first and the second division of meiosis, but it does not happen. This explains how the chromosome number is halved during meiosis. One diploid nucleus, in which each chromosome consists of two chromatids, divides twiceto produce four haploid nuclei in which each chromosome consists of one chromatid.

Locus

Each gene occupies a specific position on the type of chromosome where it is located. This position is called the locus of the gene. The position of a gene on a chromosome. Maps showing the sequence of genes along a chromosome can be produced by crossing experiments, but more detailed maps can now be produced when the genome of a species is sequenced. If two chromosomes have the same sequence of genes (not the same genes, but same sequence as in loci of genes on a chromosomes, can be different alleles) then they are homologous. All the genes on a chromosome are part of one DNA molecule.

47xx - trisomy 45X0-monosomy

Extra chromosome (47) Missing one (45)

Sex Determination

Females have two X chromosomes and pass on one in each egg cell. So all offspring inherit an X chromosome from their mother. The gender is dtermined at the moment of fertilization by one chromomse carried in the sperm. This can be either a X or a Y crhomosome. When sperm are formed half contain the X chromosome and half the Y. Daughters inherit the X chromosome and son's the Y.

Fertilization and genetic variation

Fusion of gametes from different parents promotes genetic variation The fusion of gametes to produce a zygote is a highly significant event both for individuals and for species. - It is the start of the life of a new individual. - It allows alleles from two different individuals to be combined in one new individual. - The combination of alleles is unlikely ever to have existed before. - Fusion of gametes therefore promotes genetic variation in a species. - Genetic variation is essential for evolution.

Zygote

Fusion of gametes results in a diploid zygote (there are two alleles of each gene, they may be the same allele, in the nucleus of each zygote).

X chromosome

Has many genes that are essential in both males and females, thus all humans must have at least one x chromosome.

Co-dominant alleles

Have joint effects. When together they are both expressed meaning that a different appearance of the phenotype results.

Haploid

Have one set of chromosomes (sex cells) Haploid nuclei: one chromosome of each pair. One chromosome of each type. 1 full set of chromosomes in its species. Haploid nuclei in humans contain 23 chromosomes. E.g. Gametes are sex cells that fuse together during sexual reproduction. They are haploid cells (haploid nuclei).

Diploid

Have two sets of chromosomes (all sets of autosomes) and 1 sex chromosomes (both set from mom and dad) Has two sets of chromosomes of eachtype (diploid nuclei: possesses pairs of homologous chromosomes). Diploid nuclei in humans contain 46 chromosomes. E.g. When haploid gametes fuse together during sexual reproduciton they form one diploid zygote. Upon division bymitosis more cells with diploid nuclei are produced. Many plants and animals consist entirely of diploid cells (excluding the cells which produce gametes for sexual reproduction). At least two copies of every gene in a diploid nucleus, except for genes on the sex chromosomes. Advantage of this: - Effects of harmful recessive mutations can be avoided if a dominant allele is present. - Organisms are often more vigorous if have two different alleles of genes instead of merely one. This is known as hybrid vigour.

Homologous Chromosomes

Homologous chromosomes carry the same sequence of genes but not necessarily the same alleles of those genes. However, the chromosomes are generally not identical to each other as at least for some of the genes on the chromosome the alleles are different. E.g. one person has curly hair one has straight hair. On one person's gene there is a genetic sequence for curly hair and on the other person's on the same gene genetic sequence for straight hair. Although the gene codes for different hair, they are still found on the same position of the specific type of chromosome (same sequence), they merely possess different alleles. Two eukaryotes of the same species are thought to possess homologous chromosomes (the chromosomes in one eukaryote are expected to each be homologous to at least one other chromosome in the other species). This allows for interbreeding.

Fertilization

In eukaryotic organisms sexual reproduction involves the process of fertilization (Union of two sex cells or gametes, usually from two different parents). Fertilization doubles the number of chromosomes each time it occurs. This would result in a doubling of chromosome number after every geneation, so the number is halved at some stage in the life cycle (meiosis).

Bacterial chromosomes

In most prokaryotes, there is one chromosome, consisting of a single strand of circular DNA containing all the genes needed for the basic life functions of the cell. DNA in bacteria is not associated with proteins (pg. 149) so it is referred to as naked. As there is only one chromosome present in a prokaryote there is generally also only one copy of each gene present (there are briefly two copies before binary fusion - DNA is replicated).

Chiasmata formation

Increased stability of bivalents at chiasmata as well as an increase genetic variabiltity if crossing over occurs. Crossing over exchanges maternal and paternal DNA. Further crossing over can occur multiple times and between different chromatids within the same homologous pair. Crossing over can also decouple linked combinations of alleles leading to independent assortment.

Gene

Is a heritable factor which consists of a length of DNA and influences a specific characteristic A portion on a DNA sequence that codes for a specific characteristic. Chromosomes possess many different genes. Genes are linked in groups and each group corresponds to one of the types of chromosomes in a species. E.g. in humans there are 23 groups of linked genes and 23 chromosomes

E. Coli

Lives in water and large intestine. In large intestine helps breakdown cellulose (which provides vitamin K aiding in blood clotting) Can't ingest may make sick

Chromosome numbers

Members of a species generally possess equivalent numbers of chromosomes. Organisms with different numbers are likely unable to procreate. The number of chromosomes can change through evolution (e.g. fusion ---> decrease, split of chromosomes ---> increase) Also mechanisms that can double chromosome number (rarer) Generally chromosome numbers remain unchanged.

Multiple alleles

More than one allele for one gene

Mutation 2.0 + Causes

New alleles are formed from other alleles by gene mutation. Mutation is a random change to the base sequence of a gene. Two factors can increase the rate of mutation: 1. Radiation if it has enough energy to cause chemical changes to DNA (such as gamma rays and alpha particles from radioactive isotopes, short-wave ultraviolet radiation, and X-rays are all mutagenic. 2. Some chemical substances cause chemical changes in DNA and so are mutagenic. Examples are enzo[a]pyrene and nitrosamines found in tobacco smoke and mustard gas.

Chi-squared

Null: Unlinked - Assort Independently Alternate: Linked - Do Not Assort Independently

Mutation

One way through which new alleles are formed. New alleles are formed from gene mutations which occurred on existent alleles. Mutations are random changes to the base sequence of DNA/RNA. The most significant type is a base substitution. A mutation on an allele which developed over millions of years by evolution is most likely not beneficial. Almost all mutations are thus neutral or harmful. Some are even lethal --> cell death in mutated cell. Mutations in body cells are annihilated when an individual perished. But mutations in cells that develop into gametes may be inherited by offspring potentially resulting in a genetic disease.

Hemaphrodite

Plants such as peas are hemaphrodites meaning that they can produce both male and female gametes. Therefore, regardless of whether a green pea plant pollinates a red pea plant or a red pea plant pollinates a green pea plant, in this reciprocal cross the results will always be the same, this is not the case with animals.

Binary fusion

Process by which bacteria cells asexually reproduce

Kinetechore

Protein structure that forms at the centromere region and attaches chromosomes to the spindle fiber (during cell division).

Segregation

Separation of alleles into different nuclei is called segregation. During gamete formation/meiosis. Breaks up existing combinations of alleles in a parents and allows new combinations to form in the offspring.

Random orientation of bivalents

The attachment of the spindle microtubules is not the same as in mitosis. The principles are these: ● Each chromosome is attached to one pole only, not to both. ● The two homologous chromosomes in a bivalent are attached to different poles. ● The pole to which each chromosome is attached depends on which way the pair of chromosomes is facing. This is called the orientation. ● The orientation of bivalents is random, so each chromosome has an equal chance of attaching to each pole, and eventually of being pulled to it. ● The orientation of one bivalent does not affect other bivalents. The consequences of the random orientation of bivalents are discussed in the section on genetic diversity later in this topic.

Asexual life cycle

The offspring have the same chromosomes as the parents so they are genetically identical.

Crossing over and random orienation promotes genetic variation

The unpredictable traits that are inhereited from each parent is often due to meiosis. Every gamete produced by a parent has a new combination of alleles - meiosis is a source of endless genetic variation.

Sexual life cycle

There are differences between the chromosomes of the offspring and the parents so there is genetic diversity.

Independent assortment

This is due to the random orientation of pairs of homologous chromosomes in meiosis I. The pole a chromsoome moves to depends on its orientation which is not and does not affect other's orientation. Thus this is independent orientation. If organism is heterozygous for a gene, then in its cells one chromosome in a pair will carrry one allele of the gene and the toehr chromosome will carry the other allele. The orientation of the pari of chromosomes determines which allele moves to which pole. Each allele has a 50% of moving to a particular pole. Because ther eis random orientation of chromosome paris, the change of two alleles coming together in the same pole is 25%.

Homologous pairs

Two chromosomes have identical gene placement (same spot same order) but there may be differences in alleles (different sequence/different form of a gene).

Trisomy

a condition in which an extra copy of a chromosome is present in the cell nuclei, causing developmental abnormalities.

Gamete

a mature haploid male or female germ (Sex) cell that is able to unite with another of the opposite sex in sexual reproduction to form a zygote. Sperm cells are more motile than egg cells, while egg cells possess a larger volume than sperm cells. Parents pass genes on to their offspring in gametes. Gametes contain one chromosome of each type and so are haploid. The nucleus of a gamete therefore only has one allele of each gene.

Pure bred

are homozygous, of a dominant or recessive trait (2 dominant or 2 recessive)

Meiosis I

havles chromosome number in cell. Because one chromosome of each type moves to each poole, both of the two nuclei formed in the first division of meiosis contain one of each type of chromosome, so they are both haploid.

Hybrid

heterozygous, one allele (dominant) one allele recessive (one allele from dad and a different one from mom)

Chiasmata

holds two chromosomes in each bivalent together, but these slide to the endoof the chromosomes and then the chromosomes can separate. This separationof homologous chromosomes is called disjunction.

Genome size

how many base pairs we have More chromosomes does not mean larger genome, as could be smaller chromosomes, as could be broken off recombining of genes, creates new species, but still less complex Genome size can only be shown through number of nucleotide bases could merely be rearranged not increased genome size

There are a number of ways in which meiosis I differs from mitosis and meiosis II (No crossing-over):

i) sister chromatids remain associated with each other ii) the homologous chromosomes behave in a coordinated fashion in prophase iii) homologous chromosomes exchange DNA leading to genetic recombination iv) meiosis I is a reduction division in that it reduces the chromosome number by half. The processes that result in the creation of genetic variety of gametes are initiated in meiosis I. The segregation of homologous chromosomes occurs during anaphase I resulting in two haploid cells, each with only one copy of each homologous pair.

Polygenic

multiple genes contribute to phenotype More genes more alleles (continuos variation vs discrete) Continuous variation: e.g. concentration or temperature, any value basically For traits, looking at things like skin color, several different shades (eye color, hair color, height, e.g) Not just one gene, many genes that contribute to it --> continuous variation The more gene contribute to the outcome, the more continuous the variation is (usually there is an environmental aspect to it as well) --> height sleep, nutrition, altitude,

Number of centromeres =

number of chromosomes (diploid means there are two sets of chromosomes for each type of chromosome)

n

number of haploid chromosomes

single nucleotide polymorphism (SNP)

positions in a gene where more than one base are present. Several SNPs can be present in a gene but even then the alleles of a gene differ by only a few bases. A DNA sequence variation resulting from the variation of one nucleotide base. At least 1% of the population must possess this alteration in order to be considered a single nucleotide polymorphism. A single base-pair site in a genome where nucleotide variation is found in at least 1% of the population. A DNA sequence variation that occurs when a single nucleotide (adenine, thymine, cytosine, or guanine) in the genome sequence is altered and the particular alteration is present in at least 1% of the population. Generally all base sequences shared among humans which gives us unity. But are mutations such as single nucleotide polymorphisms which give us diversity.

Meiosis involves a halving of the chromosome number; thus it is known as a...

reduction division

Highly repetitive sequences of DNA

satellite DNA

Sex chromosomes determine...

sex x chromosomes: relatively large and has its centromere near the middle y chromosome: much smaller and has its centromere near the end

Karyogram

shows the chromosomes of an organism in homologous pairs of decreasing length. Chromosomes are visible during mitosis, best view during metaphase. Stains are used to make the chromosomes show up. Some stains give each chromosome type a distinctive pattern. If dividng cells are stained and placed on a microscope slide and are then burst by pressing on the cover slip, the chromosomes become spread. Often they overlap each toher, but with careful searching a cell can usually be found wtih no overlapping chromosomes. A micrograph can be taken of the stained chromosomes. Chromosomes are arragned according to size and strucutre. The position of the centromere and the pattern of banding allow chromosomes that are of a different type but similar size to be distinguished. Most cells are diploid, so chromosomes often in homologous pairs.

Which type of cell is used to create a karyotype of a fetus?

somatic cell

kinetechore fibers

spindle fibers that will line up and then separate chromosomes (move to the poles) The main generators of force that move chromosomes during mitosis.

Genome

whole of genetic informaiton in an organism. DNA consists of genetic information, thus the genome is the entire base sequence of each DNA molecule. In humans the genome consists of the 46 chromosomes in all cells and the DNA in the mitochondrion (this is the case for most organisms).