Topic 7-Genetics, populations, evolution and ecosystems

What is meant by the term genotype?

What is meant by the term genotype?

• Genetic constitution of an organism

What is meant by the term phenotype?

• The expression of this genetic constitution (genotype) and its interaction with the environment

What are alleles and how do they arise?

• Variations of a particular gene (same locus) → arise by mutation (changes in DNA base sequence)

How many alleles of a gene can be found in diploid organisms?

● 2 as diploid organisms have 2 sets of chromosomes (chromosomes are found in homologous pairs)

○ But there may be many (more than 2) alleles of a single gene in a population

Describe the different types of alleles

• Dominant allele —> Always expressed (shown in the phenotype)

• Recessive allele —> Only expressed when 2 copies present (homozygous recessive) / NOT expressed when dominant allele present (heterozygous)

• Codominant alleles—>Both alleles expressed / contribute to phenotype (if inherited together)

What is meant by the terms homozygous and heterozygous?

• Homozygous- Alleles at a specific locus (on each homologous chromosome) are the same

• Heterozygous- Alleles at a specific locus (on each homologous chromosome) are different

What do monohybrid and dihybrid crosses show?

● Monohybrid cross - inheritance of one phenotypic characteristic coded for by a single gene

● Dihybrid cross - inheritance of two phenotypic characteristics coded for by two different genes

Worked example: monohybrid cross (basic)

The dominant allele for the grey gene (G) in horses results in a coat colour turning progressively grey.

The recessive allele (g) results in the normal coat colour being maintained (non-grey phenotype).

A non-grey female is crossed with a heterozygous male.

Draw a genetic diagram to show the expected ratio of phenotypes in the offspring.

Worked example: monohybrid cross with multiple alleles

In one snail species, shell colour is controlled by a gene with 3 alleles. The shell may be brown, pink or yellow. The allele for brown (CB) is dominant to the other two alleles. The allele for pink (CP) is dominant to the allele for yellow (CY).

A cross between two pink-shelled snails produced only pink-shelled and yellow-shelled snails.

Use a genetic diagram to explain why.

Worked example: monohybrid cross with codominance and multiple alleles

The inheritance of the ABO blood groups in humans is controlled by three alleles of a single gene, IA, IB and IO. The alleles IA and IB are codominant, and the allele IO is recessive to IA and recessive to IB.

Two heterozygous parents plan to have a child. One has blood group A and the other has blood group B.

Use a genetic diagram to show all the possible genotypes and the ratio of phenotypes expected in their offspring.

Worked example: monohybrid cross using a pedigree diagram

The figure shows the inheritance of a genetic disorder in a family.

Person 7 and person 8 plan to have another child. Determine the probability that the child will be a male who has the disorder. Use the symbol H for the dominant allele and h for the recessive allele.

In this example, we first have to use the pedigree diagram to work out whether the disorder was caused by a dominant or recessive allele. Since two parents without the disorder had a child with the disorder, it is caused by a recessive allele.

We also need to use the diagram to work out the genotypes of 7 and 8 (parents). Since 7 has the disorder, he must be homozygous recessive. 8 doesn’t have the disorder so must have at least one dominant allele. Since 7 and 8 have a child with the disorder, 8 must be heterozygous to have passed on the recessive allele.

The allele for colour blindness is carried on the ‘X’ chromosome.Colour blindness is more common in males than in females.

Why?

● Females (XX) have 2 alleles → only express recessive allele if homozygous recessive / can be carriers

● Males (XY) have 1 allele (inherited from mother) → recessive allele always expressed

What is a sex-linked gene?

• gene with a locus on a sex-chromosome (normally X)

A single gene that’s carried on the X chromosome controls the presence of hair on the skin of cattle. Its dominant allele causes hair to be present and its recessive allele causes hairlessness.

A male and a female with hair have two hairless female offspring.

What is the probability that their next calf born is hairless?