Extensions to Mendel's Principles

”sex-linked” characteristics

the inheritance of characteristics coded on sex chromosomes

Pattern of inheritance of sex-linked characteristics is _________ from autosomes

different

Most sex-linked characteristics are __-linked

X

Pattern of sex-linked inheritance was first described in __________ (2 forms of X determine whether white- or red-eyed)

Drosophila

If X⁺ is dominant over X^W, a cross between a white-eyed female (X^WX^W) and a red-eyed male (X⁺Y) will produce…

0% white-eyed females and 100% white-eyed males

If X⁺ is dominant over X^W, a cross between a red-eyed female (X⁺X^W) and a white-eyed male (X^WY) will produce…

50% white-eyed offspring

Sex-linkage (XX-XY): eye color alleles only on X chromosome (X-linked)

Females = homozygous or heterozygous, males = hemizygous

• Females inherit an X from father, males inherit a Y from father

• Males and females inherit one of two Xs from mother

X and Y have different _____

genes

X and Y pair as “_________” during meiosis

homologs

ZZ-ZW Sex-linkage

similar to XX-XY but reversed (females hemizygous, males homo/hetero)

each ____ cell expresses an opsin protein involved in detecting red, green, or blue light (can sense color); mutations in opsin genes effect ability to see colors

cone

Red and green opsin genes are on X chromosome, making color-blindness __-_____

X-linked

human X-linked traits

eye color

color blindness

hemophilia (blood clotting factors)

muscular dystrophy

is the X or Y chromosome shorter (64 genes)?

Y

Y chromosome genes are associated with…

male development and fertility (most not well understood)

Y and X evolved from ________

autosomes

Palindromic sequences

Extensive repetitive & palindromic DNA sequences

important for internal recombination (maintain chromosome)

Phenotype is often associated with amount of ______ produced

protein

Two copies of chromosome will typically ‘______’ the amount of protein

double

Autosomes and XX females have two copies of each chromosome, whereas males have only one X. Thus, for genes on X, females would have twice the amount of ‘____ ______’

gene dosage

To compensate for the doubled amount of gene dosage in females, one X chromosome is randomly __________ early in development (except some X genes escape, and not all diploid organisms compensate)

inactivated

Dominance

allelic interaction

Dominance does not relate to patterns of inheritance; it is an interaction between…

the products of genes

_________ can occur at different levels because phenotype can be observed at different levels

Dominance

genetic __________ often have a molecular phenotype, a cellular phenotype and a physiological phenotype

diseases

alleles may be dominant at one level and ____________ at another level

co-dominant

Phenotype of __________ determines the type of dominance

heterozygote

Barr body

inactivated, highly condensed X chromosome

Calico cat coat

example of X chromosome inactivation (indicates which was inactivated)

Complete dominance

Heterozygote has same phenotype as one of homozygotes

Incomplete dominance

Heterozygote has a phenotype intermediate between homozygotes

Incomplete dominance results in a _______ phenotypic ratio from a heterozygous cross

1:2:1

Genotypic and phenotypic ratios are the same in incomplete dominance because each genotype has its own __________

phenotype

If an F1 eggplant is used in a test cross with a homozygous recessive plant, what proportion of the progeny from this cross will be white?

1/2

Codominance

when both alleles are expressed, retaining their own identity; ie., a heterozygote simultaneously expresses phenotypes of both homozygotes

AB blood alleles

ABO type depends on carbohydrate chains added to cell surface proteins and lipids

Antigen and thus blood type are determined by polymorphic glycosyltransferases (codominance)

Penetrance and expressivity dominance

Dominant allele may not always show expected phenotype due to effects of other genes and environment

Penetrance

% of individuals with a particular genotype that express expected phenotype (100% = dominant phenotype is seen in every indiciudal with that allele; 50% = only half of those with dominant allele show that trait)

Expressivity

the degree to which a trait is expressed

penetrance vs expressivity

penetrance: is trait present or not?

expressivity: to what degree is trait expressed?

Example of penetrance and expressivity

polydactyly in humans: caused by dominant allele, but some individuals with dominant allele do not have the phenotype (penetrance), and number of extra digits varies (expressivity)

lethality dominance

Some alleles are lethal in the homozygous state —> death during development

Because some of the progeny die due to lethality dominance, the expected _____ is altered

ratio

mouse coat color example of lethality dominance

Y is recessive lethal but color dominant

A cross between two green corn plants yields 2/3 green progeny and 1/3 yellow progeny. What is the genotype of the green progeny if the dominant allele is recessive lethal and color dominant?

Ww (WW individuals died)

A cross between two green corn plants yields 2/3 green progeny and 1/3 yellow progeny. What is the genotype is lethal?

WW

multiple alleles

when there are more than two alleles in population (still only two alleles in diploids)

Human ABO blood type shows ________ ________ (and co-dominance)

multiple alleles

in Mendel’s dihybrid crosses, the genes at each locus:

a) independently assorted during meiosis (to produce 9:3:3:1 phenotype)

b) were independent in their influence on phenotype (seed shape, seed color)

Frequently, genes at two loci do not act __________ on phenotype

independently

the effect of genes at one locus often depends on presence of gene(s) at different locus

Products of genes at different loci interact to produce new, unpredicted phenotype

Example of a gene interaction that produces novel phenotypes

genes at two loci interact to produce fruit color in peppers

gene interaction with epistasis

one gene masks (hides) the effect of a gene at another locus

example of gene interaction with epistasis

Lab coat color depends on pigment production (‘first’ locus), then pigment deposition (‘second’ locus)

Gene actions must happen in sequence: pigment must be produced before it can be deposited

Black is dominant to brown (BB bb) = hypostatic gene

Deposition is dominant to non-deposition (EE ee) = epistatic gene

recessive epistasis

2 recessive alleles are required at the second locus to mask the first locus

Sex-__________ characteristics are determined by autosomal genes, but expressed differently depending on sex of the individual

influenced

example of a sex-influenced characteristic

goat beard presence depends on allele (B^b) that is dominant in males and recessive in females

sex-limited trait

expressed in only one sex

Cytoplasmic inherited traits are present in both males and female but inherited only from _______

mother

Cytoplasmic inheritance

Characteristics encoded by genes in cytoplasm (mitochondrial and chloroplast DNA)

Different cells and different individual offspring will have range of phenotypes due to random __________ of heteroplasmic organelles in cytoplasmic inheritance

inheritance

Genetic maternal effect

Genotype of mother determines phenotype of offspring (neither male parent nor offspring’s genotype affects offspring’s phenotype)

In cases of genetic maternal effect, the paternal genotype only becomes apparent in ___ because the father’s genes contribute to genotype of daughters

F2

In a species that exhibits the genetic maternal effect, which females will produce sinistral offspring (s⁺ = dextral dominant)?

only ss

genomic imprinting

the expression of an allele is determined by the parent from which it was inherited

Maternal genomic imprinting

the allele inherited from the mother is silenced

Paternal genomic imprinting

the allele inherited from the father is silenced

Genomic imprinting depends on ____ ___________ and is a form of epigenetics

DNA methylation

Example of environmental effects on phenotype

temperature-sensitive alleles in rabbits (rabbits raised in colder climates have more dark fur)

Continuous characteristics (quantitative characteristics)

show continuous variation (e.g., human height) and may also be influenced by environmental factors

continuous characteristics often involve genes at many loci (________ characteristics)

polygenic