7. Heredity in Animals

Introduction to genetics

By the end of this session you will be able to:

• Describe what is meant by the term ‘genetics’.
• Describe the principles behind inheritance via sexual reproduction.
• Explain key words and phrases.

DNA

DNA in the cell

What do we mean by genetics?

Genetics is the study of genes and their effects.

A gene is a section of DNA that codes for a particular characteristic.

Chromosomes are made up of hundreds of genes.

What do we mean by genetics?

• We are studying the effect that DNA has on the organism that it belongs to and how that DNA is inherited by offspring and the effect that this has.

Chromosome numbers

Chromosomes from a normal human male.

Chromosome numbers

Different animals have different numbers of chromosomes:

60

Chromosome numbers

Different animals have different numbers of chromosomes:

54

Chromosome numbers

Different animals have different numbers of chromosomes:

64

Chromosome numbers

Different animals have different numbers of chromosomes:

78

Chromosome numbers

Different animals have different numbers of chromosomes:

38

Chromosome numbers

Different animals have different numbers of chromosomes:

56

Chromosome numbers

Different animals have different numbers of chromosomes:

88

Chromosome numbers

Different animals have different numbers of chromosomes:

2 (female), 1 (male)

Chromosome numbers

Different animals have different numbers of chromosomes:

80

Chromosome numbers

In a real quirk of nature:

• increasing the number of chromosomes does not seem to increase either the size of the animal or its complexity.

Inheritance

Sexual reproduction is good for a species because it increases the diversity seen within the species.

But how?

Basic reproduction

Female egg (ovum) + Male sperm = new life.

Basic reproduction

Chromosomes contain genes (instructions for a particular characteristic), the offspring receives one set of instructions from the mother and one set of instructions from the father.

When we look at inheritance, we look at how these instructions (genes) are interacting.

What is a Haploid cell?

- only half the number of chromosomes (i.e. a sex cell has 23 pairs of chromosomes).

What is a Diploid cell?

- has the full number of chromosomes (46 pairs).

Genetics

Genotype – total genetic make-up of an individual.

Phenotype – observable looks and behaviour of an individual. An interaction between genotype and environment.

Genetic information

Locus – the gene's own location on the chromosome

Allele – different forms of the same gene, blue or brown eyes.

Meiosis

This is a type of cell division that results in four daughter cells each with half the number of chromosomes of the parent cell, as in the production of gametes.

Deoxyribonucleic Acid

DNA (short for deoxyribonucleic acid). The material containing the genetic instructions used in development and function of an organism. DNA is arranged in double helix–shaped strands.

Mutation of genes

A mutation is a change in a gene, and it gives rise to new alleles at a particular locus.

It’s the way we get new genetic variation and can form new breeds, seen best in dogs. The Bedlington terrier is a woolly version of a whippet, and miniature cattle and horses have come from mutations.

Mutation rates for a gene are usually very low, e.g. one in a million.

Mutations can be hastened by radiation, certain chemical treatment, abnormally high or low temperatures or by other genes carried by the organism.

Naming Genes

We use letters for this and there is an international convention to avoid confusion. Some genes are called after the person discovering them, e.g. Inverdale gene in Texel sheep.

The capital letter is used for the dominant allele and the lower-case letter for the recessive.

Example:
P = polled allele in cattle, p = horned allele

Genotype and phenotype

• W = white wool in sheep
• w = black wool colour

When we describe the animal, as it has a ‘diploid’ or double state of the allele, two letters are used. Example:

• PP = polled cow
• pp = horned cow
• WW = white sheep
• ww = black sheep

Heterozygous and Homozygous pairs of chromosomes

A homozygote is an individual that has identical alleles at a given locus. E.g. PP or pp.

A heterozygote has non-identical alleles at a given locus. E.g. Pp.

When you mate a polled bull (PP) to a polled cow (PP) – the offspring are all polled (PP).

When you mate a horned bull (pp) to a horned cow (pp) – the offspring are all horned (pp).

When you mate a polled bull (PP) to a horned cow (pp) the offspring are all polled (Pp). The dominant big P allele stops the small p allele from being expressed.

Note these are heterozygous as they have non-identical alleles and consequently won’t breed true.

Sex-linked Chromosomes

Sex-linked is a trait in which a gene is located on a sex chromosome. The term generally refers to traits that are influenced by genes on the X chromosome. This is because the X chromosome is large and contains many more genes than the smaller Y chromosome.

In a sex-linked disease, it is usually males who are affected because they have a single copy of the X chromosome that carries the mutation. In females, the effect of the mutation may be masked by the second healthy copy of the X chromosome.

Males : XY

Females : XX

Lethal genes

This is a gene that in some conditions, e.g. if it is homozygous, may prevent development or cause the death of an organism or its germ cells. The gene can also be called the ‘lethal factor, the lethal mutant, or a lethal mutation’.

Examples:

• Dropsical (bulldog) calves in cattle.
• Imperforate anus in calves, sheep and pigs.
• Hydrocephalus in cattle, sheep and pigs.
• Cleft palate (general).
• Nakedness in poultry.

Estimated Breeding Value

Estimated Breeding Values (EBVs) are measurements of genetic potential, which can be used to assess an animal’s breeding merit for a specific trait.

The value is given a + or a - and is linked to important traits.

Traits in Rams

Trait

Traits in Beef Bulls

Conclusion

At the end of this session you should be able to:

• Describe what is meant by the term ‘genetics’.
• Describe the principles behind inheritance via sexual reproduction.
• Explain key words and phrases used in genetics in farming.