What is dihybrid inheritance? • The study of genetic crosses involving two characteristics or two genes is called dihybrid inheritance. • In this section we will cover: • 1) dihybrid inheritance with complete dominance • and • 2) effects of linked genes and crossing over on dihybrid inheritance
Learning by examples • Example 1 HORSES. • Black coat colour in horses is produced by a dominant allele B, and chestnut or red by the recessive allele b. • The trotting gait (moving legs on opposite sides of the body together) is due to a dominant allele T. Pacing gait (moving the two legs on the same side of the body together) is due to the recessive allele t.
If a homozygous black pacer is mated with a homozygous chestnut trotter, what will the appearance and genotype of the F1 (first) generation?
What will be the genotypes and phenotypes of the offspring of a cross between two heterozygous trotter (BbTt) horses?
Genotypic ratio of offspring:- • BBTT : BBTt: BBtt: BbTT: BbTt: Bbtt: bbTT: bbTt: bbtt • 1 : 2 : 1 : 2 : 4 : 2 : 1 : 2 : 1 • Phenotypic ratio of offspring:- • Black coat trotters: black coat pacers: chestnut trotters: chestnut pacers • 9 : 3 : 3 : 1
Meiosis • Before we look at linked genes and crossing over we need to understand the process of meiosis.
Meiosis • Meiosis is the cell division that produces gametes (sex cells) for the process of sexual reproduction. • In animals the gametes are ova and sperm and are produced in the testes and ovaries. • In plants the gametes are ova and pollen produced in the ovary and anther.
Halving the number of chromosomes • If the life cycle contains sexual reproduction, there must be a point in the life cycle when the number of chromosomes is halved. • This means that the gametes would contain only one set of chromosomes (haploid number) rather than the diploid number.
Meiosis involves two division • In the first division (Meiosis I) also known as the reduction division, the homologous chromosomes pair up. This is synapsis. One chromosome from each pair of homologous chromosomes separates into the two separate intermediate cells produced at the end of the first division. • The second division (Meiosis II) is like mitosis. Chromatids separate into separate cells.
Summary • Each cell at the end of the first division produces two daughter cells to give a total number of four cells at the end of the second division. • Each of the daughter cells has half the number of chromosomes found in the original reproductive cell.
Segregation • Segregation is the separation of the alleles for each gene into separate gametes. • This causes some variation in the gametes; however, gametes unite during fertilisation to restore the chromosome number to diploid. • Fertilisation brings another set of alleles into the offspring, which results in greater variation between offspring.
Independent Assortment • Independent assortment takes place when homologous chromosomes line up on the equator of the cell during the first division of meiosis. • The way each pair line up together is independent of how the other pairs line up.
Independent Assortment (contd) • This adds much variation to the gametes, as, for example, a cell with 2 homologous [airs can produce gametes with 4 different combinations of chromosomes. • A cell with 3 homologous pairs can produce gametes with 8 different combinations of chromosomes and so on.
Crossing over • Crossing over occurs when the homologous pairs line up together at the beginning of meiosis. • The chromosomes have already replicated, which enables two non-sister chromatids to lie across each other, break off and join the other chromatid. • During this process, some alleles are transferred from one of the chromosome pair to the other of the pair.
Crossing over contd. • Crossing over makes a significant contribution to genetic variation between gametes, because it moves alleles from one chromosome to another to produce gametes containing combinations of alleles that would not be possible without crossing over.
Summary • Segregation separates the alleles unchanged from how they were inherited • Independent assortment mixes chromosomes inherited from the female and the male parent. • Crossing over contributes most to genetic variation because it mixes alleles that were inherited from the female parent with alleles that were inherited from the male parent.
Linked genes • Linked genes are genes that are on the same chromosome but code for different characteristics. • Alleles for linked genes are inherited together, so the alleles do not segregate in the same way as alleles for genes that are on different chromosomes segregate.
Linked genes in chickens contd • The four chicken genes on chromosome 1 are linked and inherited together so instead of many different gametes produced there are only two.
Linked genes in chickens contd. • If these genes were on separate chromosomes, an individual heterozygous for these four genes would be able to produce gametes with 16 different combinations of alleles. • Crossing over during meiosis can change the expected combinations of alleles for linked genes.
Linked genes in chickens contd. • For example, if crossing over ever occurred between the O and the P gene on the homologous pair shown, new unexpected combinations of alleles would occur in the gametes. • Instead of there being only two gametes with different combinations, there would be four.
Dihybrid inheritance of linked genes • Linkage of genes reduces the number of possible combinations of alleles in the gametes compared with the number of combinations from alleles of genes that can assort independently.
Dihybrid inheritance of linked genes example • The following crosses are between parents heterozygous for both egg colour and feather colour with homozygous recessive parents that have brown eggs and lavender feathers. • Parent genotypes: • A) Draw a punnet square to show this cross if the genes are not linked and describe the phenotypic and genotypic ratio of the offspring.
Dihybrid inheritance of linked genes example contd. • B) Genes O and L on one chromosome of the homologous pair are linked. Draw a punnet square to show the genotypes and phenotypes of the offspring.
Dihybrid inheritance of linked genes example with crossing over • If crossing over occurred between the O and the L gene in the heterozygote, the recombinant gametes carrying Ol and oL would occur with the expected OL and ol gametes. This would make the preceding linked-gene cross the same as the first ‘not-linked’ cross. • This is shown in the punnetsquare below.