Exam 1 Genetics -- BIOL 233

Monohybrid Cross

A cross between two pairs of alleles, one from a maternal lineage and another from a paternal lineage

True Breeding

A homozygous dominant parent is crossed with a homozygous recessive parent

P1

Parental generation

F1

The first generation of offspring born to the parental generation

F2

The second generation of offspring born to the first generation of offspring

Dihybrid Cross

A cross between four alleles from two parents of two different genes

Product Law

The probability of two independent events both occurring is the product of their individual probabilities

Branch Diagram

An alternative to a Punnett Square that calculates the probabilities of certain genotypes/phenotypes occuring

Punnett Square

A diagram that is used as a tool to calculate the probabilities of certain alleles being passed down from parents to offspring

Pedigree

A chart that shows the familial relationships and the genetic diseases within them, passed on or not passed-on from generation to generation

Mendel’s 1st Postulate

Unit Factors: Genes have alleles that pass on a certain type of gene from parent to offspring

Mendel’s 2nd Postulate

Dominance/Recessiveness: Some alleles are “stronger” than others, leading to a discrepancy between dominant alleles (more often to be shown when present) and recessive alleles (vice versa)

Mendel’s 3rd Postulate

Segregation: When an allele is passed from parent to offspring, it is independent of all other alleles

Mendel’s 4th Postulate

Independent Assortment: Genes are all independent of one another as they are passed from parent to offspring

Testcross

Crossing a parent with a dominant phenotype with a parent of a homozygous recessive genotype parent to determine the genotype of the parent displaying dominance

3:1 Ratio

A ratio of phenotypes made when two heterozygous parents are crossed

1:2:1 Ratio

A ratio of genotypes made when two heterozygous parents are crossed

9:3:3:1 Ratio

A ratio of phenotypes made when two heterozygous parents are crossed for TWO genes in a dihybrid cross

12:3:1 Ratio

A ratio of phenotypes made when two heterozygous parents are crossed for TWO genes in a dihybrid cross, whilst exhibiting dominant epistasis

9:3:4 Ratio

A ratio of phenotypes made when two heterozygous parents are crossed for TWO genes in a dihybrid cross, whilst exhibiting recessive epistasis

Crossing Over

The recombining of DNA during Prophase I to increase genetic diversity and prevent deterioration of the chromosomal DNA

Meiosis

The process in which created haploid sex cells

Synapsis

The locus where two nonsister chromatids intersect, and where recombination occurs during Prophase I

Bivalent

The DNA tangles of 2 pairs of sister chromatids before becoming tightly coiled chromosomes

Tetrad

The complete set of 2 pairs of sister chromatids, all linked by synapsis or chiasma

Chiasma

The locus where two sister chromatids intersect

Non-sister Chromosomes

Chromatids of opposite homologous chromosomes

Nondisjunction

An event where homologous chromosomes at synapses or sister chromatids at chiasma do not segregate as they should, leading to the creation of sex cells outside of the haploid norm

Gene Interactions

Two different genes that don’t act on eachother, but that both act on the same trait

Allele

The “versions” or “flavors” of a gene passed from parents to offspring. Two alleles, one from a maternal lineage, and another from a paternal lineage, make up a gene.

Loss-of-Function Mutation

A mutation that causes a gene to lose its function (potentially fatal)

Gain-of-Function Mutation

A mutation that causes a gene to gain its function (potentially fatal)

Incomplete Dominance

Offspring of two parents has neither the characteristics of either parents, but has a new phenotype that combines both parental phenotypes

Threshold Effect

The boundary between which an affected individual displays a phenotype and an affected individual does not display a phenotype

Codominance

Offspring of two parents display both phenotypes of parents. Blood type is a primary example of this

Recessive Lethal Allele

When found as homozygous recessive in individuals (and heterozygotes across the threshold), a particular mutation causing recessive allele kills the individual.

Dominant Lethal Allele

When found as homozygous dominant or heterozygous in individuals (but sometimes not in heterozygotes if the threshold is not crossed), a particular mutation causing dominant allele kills the individual.

Recessive Epistasis

When a gene is masked by another gene’s homozygous recessive nature

Dominant Epistasis-When a gene is masked by another gene’s homozygous dominant or heterozygous nature

Epistasis

The masking of one gene by another; when one gene has an effect at another gene’s locus

Pleiotropy

The process by which one gene affects multiple phenotypes in the individual

Penetrance

The proportion of individuals that express a phenotype

Expressivity

Everyone with a genetic mutation displays a phenotype, but to varying degrees

Temperature-Sensitive Mutations

Mutations that arise when a certain temperature threshold is met

Genetic Anticipation

As a genetic disorder is passed through generations, more copies of the genetic mutation are added, increasing the expressivity of the individual. As its passed, offspring will have more severe cases earlier in their life

Genetic Background

All other genes that may interact with the gene of focus

Organelle Heredity

The passing of genetic information from parent to offspring through mitochondria or chloroplast inheritance

Mitochondria

An organelle in both animals and plants that uses cellular respiration to transform glucose into ATP and byproducts, used by the individual to do work.

Chloroplast

An organelle only in plants that uses photosynthesis to produce sugars that the plant can use in cellular respiration.

Chloroplastic Heredity

The DNA passed from both parents to offspring within the chloroplast of the cell, not the nucleus.

Mitochondrial Heredity

The DNA passed from the maternal parent to offspring within the mitochondria of the cells, not the nuclei.

Heteroplasmy

An organelle has two or more variants of its genetic material

X-linkage

When a gene is attached to the X chromosome, which is passed from parents to offspring in a semi-more predictable manner. Oftentimes males are affected more than females, as females can be carriers if heterozygous for a particular trait.

Y-linkage

When a gene is attached to the Y chromosome, which is passed from the paternal parent to all XY male offspring

Hemizygous

XY males, as they have only 1 X chromosome compared to XX females who have 2

Sexual Differentiation

The physical and genetic differences between the male and female sexes

Heteromorphic Chromosomes

Chromosomes that don’t match (XY in humans)

Sex Chromosomes

X and Y Chromosomes

Sex Determination

The process by which sex is determined by the

Heterogametic Sex

In humans, XY males are the heterogametic sex, which are the sex with differeing sex chromosomes.

Homogametic Sex

In humans, XX females are the homogametic sex, which are the sex with a pair of the same chromosome

Turner Syndrome

When only one X is passed to offspring from parents; 45, XO Female

Huntington’s Syndrome

An example of genetic anticipation in which a mutation appears in adulthood, but as it’s passed down through generations, the mutation is copied many times over. The longer the mutation, the more intense the syndrome is and the more early unset it is. Huntington’s is the slow deterioration of nerve cells in the brain

Klinefelter’s Syndrome

When two X chromosomes and one Y chromosome are passed from parents to offspring; 47, XXY Male

Mosaic

XX individuals are heterozygous for an X-linked trait; some cells have one X disabled and some the other X. This can lead to differing traits depending if the threshold for penetrance is reached

Pseudoautosomal Regions (PAR)

The region of the Y chromosome that matches the X chromosome

Male-Specific Region of the Y (MSY)

The region of a Y chromosome that differs from the X chromosome

Sex-Determining Region of the Y (SRY)

The region of the Y chromsome linked to the developement of male sexual characteristics

Testis-Determining Factor (TDF)

The protein encoded by the SRY to synthesize the production of body cells found in the gonads: testicular tissue

Dosage Compensation

Because XX females get a “double dose” of the X chromosome, to prevent the activation of the same genes twice, one is disabled

Barr Bodies

The disabled X chromosome becomes a barr body which continues to be replicated, but is prevented from expressing any genes to phenotype

Lyon Hypothesis

Inactivation is not always of the maternal X chromosome or the paternal X chromosome, but is instead random. However, the chromosome inactivated at first will be the chromosome inactivated for every daughter, granddaughter, etc. cell thereafter

Imprinting

One copy of parental gene is expressed, while the other is suppressed

Epigenetics

Phenomena that cause change in gene expression by modifying the DNA sequence

X-Inactivation Center (XIC)

A region of DNA sequence found only on inactivated X chromosomes that causes the inactivation of the chromosome

Genic Balance Theory of Inheritance

The theory that sex is determined based on the ratio between autosomal chromosomes and X chromosomes (what drosophila follow)

Chromosomal Theory of Inheritance

The theory that sex is determined by the combination of X and Y or Z and W chromosomes (what humans follow)