Mendelian GeneticsYear 13 Biology

Revision – The Basics Monohybrid Cross

Genetic cross that considers only one characteristic.

Revision – The basics Back-cross or test cross

By looking at the phenotype, you can’t tell if an organism’s genotype is homozygous or heterozygous – do a test cross by mating your unknown genotype with an organism showing the recessive trait.

If any of the offspring show the recessive trait, the original unknown parent was heterozygous.

If no recessive trait shown up in the offspring then, it was homozygous.

Revision – The basics Dominance

1. Incomplete dominance – when one allele does not completely dominate over the other e.g. snapdragon flower. When a red flower plant is crossed with a white flower plant, the heterozygous plant is pink.

2. Codominance – same as above except the heterozygote shows both traits.

Revision – The basics Dominance

3. Overdominance – e.g. if one homozygote is tall and the other is short then the heterozygote will be even taller than the tall homozygote. Can lead to hybrid vigour.

Revision – The basics Lethal Genes

Homozygous dominant is lethal (dies as an embryo)

Instead of 1:2:1 ratio you get a 2:1 ratio

E.g. In mice yellow fur is dominant to grey. If two yellow mice mate you get ¼ YY (died), ½ Yy (yellow), ¼ yy (grey) – the homozygous dominant is lethal.

Revision – the basics Multiple Alleles

Three possible alleles for each gene. E.g. Blood groups – A, B and O A and B are codominant and they are both dominant

to O, which is recessive.Blood Group Genotype Antigen on Cell Serum agglutinin

A IAIA or IAi A Anti-B

B IBIB or IBi B Anti-A

AB IAIB A and B None

O ii none Anti-A and anti-B

Revision – the basics Dihybrid Cross

Cross between organisms with two different characteristics

Remember: Phenotype ratios for a dihybrid hetetozygous

cross 9:3:3:1 9/16 – the two dominant alleles 3/16 – one dominant + one recessive 3/16 – other dominant + other recessive 1/16 – the two recessives

Gene-Gene Interactions Situations where genes on two different

loci or on different chromosomes have an affect on each other.

1. Collaboration

2. Complementary genes

3. Supplementary genes (epistasis)

Collaboration When one characteristic is

controlled by two or more pairs of alleles.

Combinations of dominant and recessive alleles result in up to four different phenotypes

E.g. comb shape in chickens

9:3:3:1 phenotype ratio

Rose Pea

Walnut

Single

Collaboration See page 132 biozone for the cross

9/16 P_R_ two dominants = Walnut

3/16 P_rr one dominant + one recessive = Pea

3/16 ppR_ other dominant +other recessive = Rose

1/16 pprr two recessives = Single

Complementary Genes Where one dominant allele can only be

expressed in the presence of another dominant allele (of another gene pair)

E.g. white and purple flower colours – must have both P and C alleles for purple. 9:7 phenotype ratio Only two “visible” phenotypes See page 135 biozone for full cross To make purple pigment, both P and C are needed i.e.

they are complementary

Supplementary Genes (epistasis) Where one particular allele (out of 4 alleles) must be present

for the full expression of another allele. E.g. mouse fur colour

_ _ cc – albino_ _ _ C – BrownB_ C_ - black

Allele C supplements allele B 9:3:4 phenotype ratio – only 3 “visible” phenotypes See page 136 biozone for full cross.

Ratios you should knowOffspring Ratio Parents’ genotype Type of inheritance

Monohybrid crosses

3:1 Aa X Aa A dominant to a

1:2:1 Aa X Aa Incomplete or codominance

1:1 Aa X aa Back or test-cross

2:1 Aa X Aa Lethal genes (AA dies)

Dihybrid Crosses

9:3:3:1 AaBb X AaBb Heterozygous cross

1:1:1:1 AaBb X aabb Back or test cross

9:3:4 AaCc X AaCc Supplementary (epistasis)

9:7 AaCc X AaCc Complementary