Exam 1 Genetics

Homozygous

Pure-breeding organisms having a genotype consisting of two identical symbols representing two copies of an allele (AA or aa)

Heterozygous

A genetic cross between pure-breeding parents with different traits of the same gene that results in the offspring with one different allele from each parent (Aa)

Phenotype

the observable traits of an organism

Genotype

The genetic constitution of an organism

Haploid

n; one copy of each chromosome/gene; humans have 23

Diploid

2n; two copies (a homologous pair) of each chromosome/gene; humans have 46

Autosomal

all the other genes in the body that are not sex-linked.

Sex-linked (X-linked, Y-linked)

alleles that appear on X or Y chromosomes

Homogametic

2 X-chromosomes (Female: mammals)

Heterogametic

X-chromosome and Y-chromosome (male: mammals)

Pedigree analysis

an inherited trait is analyzed over the course of a few generations in one family

Chromosome

threadlike structure of DNA + protein carrying genetic information in the form of genes

Centromere

the region of the chromosome that holds the two sister chromatids together; a specialized DNA sequence on each chromosome

What do homologous chromosomes have in common?

- structural features: length and centromere position

- genes will be in the same loci (the alleles of the gene could be different but the gene that corresponds to those alleles will be the same)

How are gametes produced?

produced during meiosis from diploid cels

Locus

the physical location of a gene on a chromosome

What is genetics?

the study of genes, genetic variation, and heredity of organisms

What is heredity?

the passing of traits from parents to offspring; inheritance/biological inheritance

Blending inheritance

the theory that there was an even mix of genes from each parent; says that the phenotype of offspring should be an average of both parents

Parental generation (p)

the cross of individuals that were true breeding for different traits

Controlled genetic cross

restricting reproduction to the plants identified beforehand as a way to yield informative results

First filial generation (F1)

- result of a cross between two parents who have been true breeded for different traits (one parent is AA, other parent is aa)

- 100% of offspring in this generation should be showing the dominant phenotype and have a heterozygous genotype

Second filial generation (F2)

- result of the cross between two F1 individuals that showed the dominant phenotype

- 75% of offspring should show the dominant phenotype

- 25% of offspring should show the recessive phenotype

What is the first law of genetics?

law of segregation

What is the law of segregation?

- the two alleles for each trait will separate/segregate from one another during gamete formation and each allele will have an equal probability (1/2) of inclusion in a gamete

- random union of gametes at fertilization will unite one gamete from each parent to produce progeny in rations that are determined by chance

Genes

DNA sequence encoding for a particular trait

Alleles

Alternative forms of a gene

Dominant

phenotype when 1+ allele is present (homozygous or heterozygous); does not refer to frequency in population

Recessive

phenotype only shows if both copies of the recessive allele are present

What is mitosis used for?

only used in autosomal (haploid) cells for growth, repair, and reproduction

What is meiosis used for?

only used for diploid organisms because it is reductive cell division; two rounds of cell division, produces four gametes from one initial cell; begins after interphase

How did alleles form?

arose due to mutations of DNA; multiple mutations can lead to multiple alleles all coding for the same gene

Silent allele

a mutated allele that does not cause any functional differences and shows the wild-type phenotype

X-linked alleles

genes that are present in only X-chromosomes and not Y-chromosomes; dominant and recessive system used to determine phenotype in females but not needed for males because they only have one X-chromosome and will show whatever phenotype corresponds with their specific allele

Y-linked alleles

genes that are present in only Y-chromosome and not X-chromsosome; will never appear in females but males only need one copy for phenotype determination

X-linked recessive alleles

Males only need one copy of the recessive X-linked allele to show the phenotype; Females need two copies of the X-linked allele to show the recessive phenotype

How are sex-linked traits written?

the allele is written as superscript on the sex-chromosome (X^w or Y^w)

Reciprocal cross

a test using two crosses that are opposite of the original cross (red female x white male -> red female x white male) used to help determine if the trait is sex-linked or not.

How are X-linked dominant traits passed down?

- if the mother does not carry the dominant allele: 100% of the daughters will exhibit the trait IF the fathers have the dominant allele

- if the mother does not carry the dominant allele: sons will not exhibit the trait

- if the mother does carry the trait: both daughters and sons will exhibit the trait but the ratios depend on the father

How are X-linked recessive traits passed down?

- if the mother does not carry the allele: daughters will never exhibit the trait but may carry it IF the father has the trait

- if the mother does not carry the allele: sons will not exhibit the trait

- if the mother does carry the trait: 50% of the sons will have the trait IF the father does

- If the mother does carry the trait: the daughters may exhibit the trait but it depends on the father

- if mother is homozygous: the ratio of offspring having the trait will depend on the father

Female marker in a pedigree

circle

Male marker in a pedigree

square

What is the indication that the individual shows the phenotype for a trait in a pedigree?

the shape will be filled if the individual shows the phenotype for the trait; does not correspond to dominance and recessiveness only refers to which individuals have the trait being analyzed

How are parental generations represented in pedigrees

shapes connected by a horizontal line with a vertical line in the middle to represent the link to offspring below them

Only options considered during pedigree analysis

- Y-linked

- X-linked dominant

- X-linked recessive

- autosomal dominant

- autosomal recessive

How to determine if the trait is Y-linked

Father has the trait; only males offspring have the trait; all of the male offspring will be affected

How to determine if the trait is dominant

two parents show the trait but there is an offspring that does not show the trait; likely that the parents were heterozygous (showed the dominant trait) and the recessive trait (not being analyzed) came back up

How to determine if the trait is recessive

Two parents that do not have the trait but there is an offspring with the trait; likely that the parents were heterozygous (showed the dominant trait) and the recessive trait (being analyzed) returned in offspring

How to determine if the dominant trait is sex-linked

If the father has the trait, all of the female offspring will have the trait; male offspring will only show the trait if the mother has it

How to determine if the recessive trait is sex-linked

If the mother has the trait, all of the male offspring will have the trait; female offspring will only show the trait if the father has the trait

Tips to complete a pedigree analysis

1. write down all of the types of inheritance

2. break down each of the crossings to get a better idea of what is going on

3. attempt to look for patterns (i.e. y-linkage)

4. look for trait disappearance/reappearance in different generations; look at parents for the next generation and see if the trait comes back (for recessive traits)

5. rule out x-linked recessive if the trait does not appear in all males

Dihybrid cross

a cross between parents that were true-breeded for two different traits

F1 generation of a dihybrid cross

100% of the individuals show the phenotypes of the dominant versions of the two traits seen in the parents

F2 generation of a dihybrid cross

Phenotypes appear in a 9:3:3:1 ratio; both of the phenotypes of the parents appear and two new phenotypic combinations arise (combination of dominant and recessive phenotypes)

Product rule of probabilities

probability of two events taking place together is the product of each occuring

Independent event

probability of one event is not affected by another separate event

independent genes

unlinked genes that are inherited separately

What is the second law of genetics?

law of independent assortment

What is the law of independent assortment?

Genes for different traits are inherited independently of each other

Independent assortment of diploid organisms

different chromosomes sorted independently into gametes during meiosis; the parent diploid cell randomly passes one chromosome per homologous pair to gametes so that inheritance of one gene does not impact the others

Tetrads

pairs of homologous chromosomes that form along the metaphase plate during Metaphase 1; total number of centromeres = 2; total number of chromatids = 4; essentially two separate pairs of homologous chromosomes that come together during Metaphase 1.

Punnett Squares

predict the results of a cross for each gene separately; used with the product rule of probabilities (during a dihybrid, trihybrid, etc cross) to produce combinations and probability/ratio of each

Heteroplasmy

Presence of two or more distinct variants of DNA within the cytoplasm of a single cell

How do both phenotypic and genotypic ratios predict inheritance?

phenotypic and genotypic ratios are not a guarentee because each reproductive event is independent from one another; the ratio stays the same for each reproductive event; 3 yellow peas does not mean the 4th pea will be green if the reproductive event that led to each pea is independent from one another

Predicting number of offspring needed for a 95% probability of seeing a particular genotype or phenotype

n = 0.30/[log(1-prob success)]; n = 0.30/[log(1-prob failure)]

Polygenic inheritance

occurs when multiple genes determine the phenotype of a trait

continuous traits

determined by multiple genes (polygenic), such as height or hair color

Quantitative Trait Loci (QTLs)

stretches of DNA that are correlated with variation in a phenotypic trait; these regions contain genes, or are linked to genes, that contribute to population differences in a phenotype

How is the phenotype determined for polygenic traits determined?

based on the amount of the trait associated with the allele present in the genotype (A allele = 10uL; a allele = 1uL; AA = 20uL, aa = 2uL)

What is the cell cycle?

the life cycle that cells must pass through to replicate their DNA and divide

What are the two phases of Mitosis

Interphase and M phase

What is the M phase?

the short segment of the life cycle during which cells divide and consists of 5 stages

Prophase

chromosomes begin to condense and centromeres begin to migrate toward opposite poles of the cell, microtubules begin to form and the nuclear envelope breaks down

Metaphase

the fully condensed chromosomes begin to align so that the sister chromosomes lie on either side of the metaphase plate and microtubules from the mitotic spindle attach to the centromere of each chromosome

Anaphase

sister chromatids separate, get pulled toward opposite sides of the cell, and the separated chromosomes are now called daughter chromosomes

Telophase

the poles of the cell begin to move further apart and the nuclear envelope begins to reform

Cytokinesis of mitosis

divides the cytoplasm of the cell so it splits into two new cells by formation of cell walls in plant cells and cleavage furrow in animal cells; the partitioning of the cytoplasmic contents of the parental cell into the daughter cells

What is interphase?

The longer period between one M phase and the next; consists of 3 stages that express genetic information, replicate chromosomes, and prepare the cell for entry into M phase; the interphase events of germ-line cells does not differ from what is seen in somatic cells

G1 phase (Gap 1)

gene expression is extremely high but varies based on how many genes express their function in the body and interaction with the other cells; the duration of this phase also varies

S phase (synthesis)

DNA replication takes place and results in a doubling of the amount of DNA in the nucleus by crating two identical sister chromatids joined together to form each chromosome

G2 phase (Gap 2)

the cell continues to grow and proteins are synthesized so the cell can be prepared to enter the M phase

Karyokinesis

equal partitioning of the chromosomal material into the nuclei of the two daughter cells

Centrosome

organelles that appear and migrate during the M phase to form the opposite poles that drive cell division; contains a pair of subunits called centrioles and are the source of spindle fiber microtubules

What is sister chromatid cohesion?

produced by the protein cohesion that localized between the sister chromatids and holds them together to resist the pull of kinetochore microtubules

Kinetochore

A specialized region on the centromere that links each sister chromatid to the mitotic spindle.

What is chromosome disjunction?

the separation of sister chromatids in the first part of Anaphase and then the daughter chromosomes are ready to move to either pole of the cell during the second part of Anaphase

Cleavage furrow

begins to form in animal cells as a part of cytokinesis, which splits the cell into two daughter cells

cell wall

begins to form in plant cells as a part of cytokinesis, which splits the cell into two daughter cells

Meiosis

type of cell division that involves two successive cell divisions during the M phase and results in the production of four haploid gametes; each round of cell division consists of 5 stages

3 hallmark events of Meiosis I

1. homologous chromosome pairing: involved synapsis to align two pairs of homologous chromosomes, which results in the formation of tetrads

2. Crossing over between homologous chromosomes: the process by which genetic material from non-sister chromatids of homologous chromosomes is transferred

3. segregation (separation) of the homologous chromosomes that reduces to the haploid number

Non-sister chromatids

Chromatids from opposite members of a homologous pair. These cross over at prophase I.

Prophase I

features 5 different stages that lead to the condensation of chromosomes into homologous chromosomes as the nuclear envelope begins to dissipate, then microtubules lead pairs of homologous chromosomes to link together as tetrads; crossing over of genetic material takes place between tetrads

Metaphase I

tetrads align along the metaphase plate, pairs of homologous chromosomes are tethered to microtubules coming from opposite centrosomes

Anaphase I

microtubules begin to separate the pairs of homologous chromosomes so that they begin to move to opposite poles of the cell

Telophase I

nuclear envelope reforms around clusters of chromosomes at each pole and they begin to partially diffuse; inside of each nucleus should be a number of homologous chromosomes equal to the haploid number

Cytokinesis of Meiosis I

the cytoplasmic material begins to separate by cleavage furrow and may be uneven leading to different sized cells; this results in 2 daughter cells with equal amounts of genetic information that matches the haploid number (23 chromosomes in humans)

What is the purpose of Meiosis II?

to divide each haploid product that is formed in meiosis I

Prophase II

nuclear envelope breaks down and the centromeres begin to migrate to opposite poles of the cell to form microtubules; the chromosomes will become more visible in each daughter cell

Metaphase II

sister chromatids are attached to microtubules and aligned along the metaphase plate; single homologous chromosomes that are no longer paired together as tetrads like in Meiosis I

Anaphase II

separation of sister chromatids begins as the microtubules split the homologous chromosomes apart and pull the sister chromatids to each pole of the cell; similar to what happens in mitosis