Modes of inheritance

What is a pedigree?

• a family history showing relationships between family members

• indicates which individuals have certain genetic pathogenic variants, traits and diseases

Why are pedigrees important?

knowing an organisms pedigree allows individuals to be selected for breeding based on their desirability for specific traits and avoid other unwanted traits

What do the lines between individuals in a pedigree indicate?

Horizontal = mating

vertical = progeny

What does the shape of symbols in a pedigree indicate?

Sex

square = male

circle = female

diamond = unknown

What does a symbol filled or partially filled indicate in a pedigree?

filled = trait present

partially filled = heterozygote

What does a line through the shape in a pedigree indicate?

The individual is deceased

What do the numbers in a pedigree indicate?

roman numerals = generation

arabic numbers = individual ID in that generation

What are the two different types of traits?

Qualitative and quantitative

What are qualitative traits?

traits that can be classified

e.g., flower colour, pea shape, presence of horns

What are quantitative traits?

traits that can be measured i.e., it has units

e.g., crop yield (t/ha), plant height (cm), fleece weight (kg)

Traits can be either dominant or recessive, what is a dominant trait?

• override recessive trait/allele

• expressed in heterozygote

• 2 expressed phenotypes

These traits CANNOT skip generations and at least one parent must be affected

Traits can be either dominant or recessive, what is a recessive trait?

• only expressed in homozygous recessive

These traits CAN skip (multiple) generations and both parents must carry the allele (but not necessarily show the phenotype)

How can you identify whether or not a trait is dominant or recessive?

Based on their inheritance behaviour i.e., if it skips a generation/s or not

What are the exceptions to Mendel’s first law? I.e., the alternatives to dominant and recessive?

• partial (incomplete) dominance

• co-dominance

• additive (no dominance)

What is partial (incomplete) dominance?

• heterozygote phenotype is INTERMEDIATE of two homozygote phenotypes

• 3 phenotypes in total

• monohybrid cross (Aa x Aa)

  • genotypes: AA, Aa, aa (ratio 1:2:1)

  • 3 phenotypes (ratio 1:2:1)

for example flower colour

red (AA), pink (Aa) and white (aa)

What is co-dominance?

• heterozygote phenotype expresses both homozygous phenotypes

• monohybrid cross (Aa x Aa)

  • genotypes: AA, Aa, aa (ratio 1:2:1)

  • 3 phenotypes (ratio 1:2:1)

for example cattle colour

brown (AA), brown and white (Aa), white (aa)

What is additivity (no dominance)?

• heterozygote phenotype is the MEAN of the homozygous phenotypes

• monohybrid cross (Aa x Aa)

  • genotypes: AA, Aa, aa (ratio 1:2:1)

  • 3 phenotypes (ratio 1:2:1)

  • phenotype is QUANTITATIVE trait only

for example milk production in cattle

milk yield; A = 2L/day, a = 1L/day

AA = 4L/day

aa = 2L/day

Aa = 3L/day

What are the other modes of inheritance?

• lethal alleles

• multiple alleles

• phenocopy

• pleiotropy

What are lethal alleles?

• one homozygous phenotype does not survive

• monohybrid cross (Aa x Aa)

  • genotypes: AA, Aa, aa (ratio 1:2:1)

  • homozygous AA (survive), heterozygous Aa (survive), aa (lethal)

  • phenotype ratio = 2:1

  • no homozygous aa

What are multiple alleles for a gene?

• more than two possible alleles for a gene

• each diploid individual still only has two of the alleles but more exist in the population

for example blood group

A, B and O alleles (three alleles)

individual can be AA, AB, AO, BB, BO and OO genotype

4 phenotypes blood group A, B, AB or O

• underlying this you also have dominant and recessive (A and B are dominant over O)

BUT also have co-dominance - neither A or B are dominant over each other - they are both present

What is phenocopy?

• environmental induced phenotype, causes one genotype to mimic another genotype

• phenotypic ratios don’t necessarily correspond to the genotype

for example Himalayan coat pattern in rabbits

• black rabbit has C (at any temperature)

• Himalayan coat allele Ch (white in normal temp, black in very low temps)

What is pleiotropy?

• single gene affecting multiple traits

for example ear shape in dogs

dogs with erect ears are less susceptible to ear infections

cannot separate the traits

one gene but two traits (erect ears and susceptibility to ear infections) traits go hand in hand

What are the exceptions to Mendel’s second law?

• epistasis

• linkage

What is epistasis?

• two genes affecting one trait

for example two genes are in the same biochemical pathway; different phenotypes depending on alleles present at both genes

different types of epistasis:

1. normal

2. dominant

3. recessive

4. duplicate genes

What is normal epistasis?

• two genes affecting one trait (A and B)

• get a 9:3:3:1 ratio

What is dominant epistasis?

• two genes affecting one trait (A and B)

• gene A dominant over gene B (but only see trait B phenotype IF gene A is recessive)

• get 12:3:1 ratio

What is recessive epistasis?

• two genes affecting one trait

• recessive phenotype in gene E overrides genotype of gene B

• get 9:3:4 ratio

What is duplicate genes epistasis?

• two genes affecting one trait (A and B)

• both genes have same phenotypic trait

• only see recessive trait if both genes alleles are recessive

• ratio 15:1