Biology 20A, Chapter 12 - Inheritance, Genes, and Chromosomes

Theory of Blending Inheritance

Gametes contain hereditary determinants (genes) that irreversibly blend together when gametes fuse at fertilization

Theory of Particulate Inheritance

Each hereditary determinant or gene has a physically-distinct nature. When gametes fuse, the determinants remain intact and can be recovered

Gamete

a mature haploid male or female germ cell which is able to unite with another of the opposite sex in sexual reproduction to form a zygote.

Pea Flower

Pistil - female organ
Stamen - male organ
Ovule - gamete
Pollen - gamete

Character

an observable physical feature, such as seed shape or seed color

Trait

a particular form of a character (round or wrinkled seeds, green or yellow seeds)

True Breeding Variety

produces the identical character and traits as their parents

Parental Generation

refers to the parents (P); the first filial generation (F1) is their offspring; the second filial generation (F2) is the offspring of the offspring

Hybrid

is the offspring of a cross between two organisms that differ in one or more characters

monohybrid cross

is between organisms that differ in only one character

dihybrid cross

is between organisms that differ in two characters

dominant traits

are more abundant... because they dominate over less abundant recessive traits

Genes

The hereditary determinants

Haploid

Organisms that have one copy of each gene

Diploid

Organisms that have two copies of each gene

Alleles

Different forms of a gene. Different traits arise from these.

Homozygous

a particular gene that has identical alleles on both homologous chromosomes

Heterozygous

having two different alleles of a particular gene or genes.

phenotype

physical appearance of an organism. Determined by interaction genotype and environment.

genotype

genetic constitution of an organism

Mendel's First Law

Two copies of a gene segregate during gamete formation. Exception is when nondisjunction occurs

Probability

can be used to predict inheritance

Gene Segregation during Meiosis I

two alleles of a gene segregate
1. The locus is the location in a chromosome where the allele resides
2. Before Meiosis I, all chromosomes are replicated in S-phase.
3. Homologous chromosome pairs line-up at the metaphase plate of meiosis I, and then chro

Locus

The location in a chromosome where the allele resides

Mendel's Second Law

Different Genes assort/segregate independently. Exception is when two genes are present on the same chromosome.

Independent Assortment of alleles

Occurs during Meiosis I metaphase

Many genes have multiple alleles

New Alleles arise by mutation, which are stable inherited changes in genetic material. Wild type alleles and mutant alleles of a gene occupy the same loci or locus in a chromosome. Because one gene can have multiple alleles, the alleles can display a domi

Wild Type Allele

the allele present in the majority of the population in nature

Mutant Allele

A variant that often produces mutant phenotypes

Example of dominance hierarchy

The color of rabbit coats is determined by multiple alleles of the C gene (C, c^chd, c^h, c) which create four different phenotypes.

Incomplete Dominance

Allele dominance is not always complete. Intermediate phenotype produced.

Co-Dominance

When both alleles of a locus are expressed in the phenotype. e.g is AB blood.

Epistasis

Occurs when the phenotype of one gene depends on the prior function of another.
Example: The function of gene E (pigment delivery and deposition) is required before the function of gene B (pigment synthesis) can be observed in the phenotype.

Hybrid Vigor

Results from new gene combinations and interactions

Inbreeding

produces offspring of 'lower quality' because there is a greater chance for allelic homozygosity... and two recessive (mutant) alleles can be problematic.

Hybrids

show greater vigor in growth due to the advantages of heterozygosity. It reduces the risk of homozygocity which is deleterious in the case of mutant alleles and phenotypes

Environment Influences to Gene Expression

Light, Temperature, and Nutrition are all factors.

Penetrance

The proportion of individuals in a population with a common genotype who show the phenotype
Example: A mutant BRCA breast cancer gene causes cancer but not in all women

Expressivity

The degree or severity of genotype expression in an individual
Example: The mutant BRCA gene affects many tissues on one person, but just one in another

Some Alleles are linked

Independent assortment is not present, and alleles sort together because they are present in the same chromosome

Recombination (crossing over)

the exchange of genetic material between homologous chromosomes during meiosis

Recombination Frequency

How often two linked genes separate from each other during meiosis by chiasmata crossover event. RF of two loci in a chromosome is equal to the number of recombinant progeny divided by the total number of progeny -> expressed in centimorgans (cM), 1% RF =

Genetic Map

Physical distance between two genes in a single chromosome can be calculated from their recombination frequency

Test Cross

A cross of an organism with an unknown genotype (allele composition) with an organism of the same type that has a known allele composition.

Conjugation

In bacteria, the direct transfer of DNA between two cells that are temporarily joined (by sex pilus).

Sex Pilus

extends from one bacteria cell into another, joining their cytoplasm. DNA passes unidirectionally through the conjugation tube in the pilus

Plasmids

small circular DNA molecules that replicate separately from the bacterial chromosome. Encodes genes involved in unique metabolic pathways: antibiotic resistance or sex pilus components.

Key Concepts 1

Key Concepts 2