GAB WK2 - Mendelian Genetics in Farm and Companion Animals

trait

specific characteristic or feature of an organism that is influences by its DNA

simple trait

1 gene

clean mendelian ratios

polygenic/complex trait

2-100s genes

many loci with small additive effects

Mendel's 3 Laws

1. law of segregation

2. rule of dominance

3. law of independent assortment

law of segregation

the two alleles at a locus separate during gamete formation - each gamete carries only one

rule of dominance

one allele can mask the expression of another in the heterozygote

law of independent assortment

alleles of different genes assort independently, provided genes are on different chromosomes (or far apart on one)

2 types of genetic crosses

monohybrid cross

dihybrid cross

monohybrid cross

a cross involving a single trait eg. coat colour alone

dihybrid cross

a cross involving two traits eg. coat colour and horn growth

genotype ratio of heterozygous x heterozygous cross at one locus

1:2:1

phenotype ratio of heterozygous x heterozygous cross at one locus

3:1

test cross

always mates the unknown against a homozygous recessive (bb)

what does a 1:1 offspring ratio prove in a test cross?

unknown parent is heterozygous

what is an all dominant litter consistent with in a test cross?

unknown parent is homozygous dominant genotype

test cross issue associated with small litters

all dominant litter is not definitive of a homozygous genotype in the unknown parent

'Product Law' of Probabilities

when two independent events occur simultaneously, their combined probability equals the product of their individual probabilities

how can the law of independent assortment be shown?

dihybrid cross

heterozygous x heterozygous dihybrid cross phenotypic ratio

9:3:3:1

non-mendelian variation

any genetic pattern where traits do not match the predictable dominant and recessive ratios established by Gregor Mendel's laws

co-dominance

both alleles are expressed side-by-side

the two products have the same function, but differ in amino acid sequence

example of co-dominance

roat coat colour in cattle

incomplete dominance

both alleles are expressed as an intermediate phenotype

Crème gene in horses

crème gene dilutes red in the heterozygote, but both red and black pigment in the homozygote

pleiotrophy

single gene controlling or influencing multiple (and possibly unrelated) phenotypic traits

example of pleiotrophy

KIT gene : causes white fur, blue eye and deafness

what can mutations in pleiotrophic genes influence?

multiple unrelated traits

epistasis

when a gene variant at one locus suppresses the effect of a gene at another locus (modifier Genes)

dominant epistasis

a single dominant copy of the epistatic gene is enough to mask the other locus

recessive epistasis

the epistatic gene must be homozygous recessive to have its masking effect

example of a gene that expresses 'dominant masking'

grey gene in horses

example of a gene that expresses 'recessive masking'

yellow gene in labradors

recessive sex-linked traits in dogs

hemophilia A and B

evident in male German Shepherds, Labrador retrievers etc

KIT gene

produces white fur, blue eye and deafness

sex influenced dominance definition + example

an autosomal trait where heterozygotes can be influenced by sex hormones eg. horn growth in sheep

dorset sheep

both sexes are horned

suffolk sheep

both sexes are hornless

result of cross between dorset and suffolk sheep

F1 females hornless, males horned

sex-linked gene

gene is on a sex chromosome (X or Y)

haemophilia in dogs

to which chromosome is haemophilia in dogs linked?

X-linked

sex-influenced gene

autosomal gene, heterozygote expression depends on sex hormones

horn growth in sheep

sex-limited gene

autosomal gene, expressed in only one sex

lactation

multiple alleles

more than two alleles can exist at one locus in a population

the agouti series

set of genes that controls the distribution of black and red/yellow pigments on an animal's hair shafts

Aʸ

dominant yellow/sable

aᵂ

agouti (wild-type banded hair)

aᵗ

tan point

a

non-agouti/solid

eumelanin

black pigment on animal hair shafts

phaeomelanin

yellow/red pigment on animal hair shafts

mitochondrial inheritance

all offspring of an affected female inherit the mitochondrial trait regardless of sex but only her daughters transmit it to the next generation

pedigree clue associated with mitochondrial inheritance

affected males never pass on the trait

conditional mutations

one that only expresses its mutant phenotype under specific environmental conditions

eg. siamese cats, pigmented points + white body

allele of Himalayan gene

• responsible for Siamese marking

• regulates the stability of Tyrosinase

• temperature dependent - young cats and cats maintained at warmer temperatures are lighter than cats that are older and at colder temperatures

tyrosinase

an enzyme in the pathway of making fur pigment

multiple mutation in the same gene

changes structure (and subsequently the function) of a gene

effect of multiple mutation in the same gene over time

many mutations accumulate - leads to presence in a population of alternative alleles at a specific locus

how many mutations are there in the KIT gene?

70