Dihybrid Crosses and Genetic Traits

Dihybrid Crosses

• Dihybrid crosses involve studying two pairs of alleles, focusing on two traits simultaneously.
• Monohybrid crosses focus on only one trait determined by a pair of alleles, unlike dihybrid crosses.

Understanding Alleles and Traits

• Each trait is determined by a pair of alleles, represented by letters such as 'H' for hair presence in guinea pigs.
• Cats, like guinea pigs, display traits, such as the presence of hair (represented by 'H') or behaviors like liking sinks.
• Two recessive alleles ('hh') are required for a cat to be hairless, while one or two dominant alleles ('H') result in hair.
• Liking sinks is presumed to be a dominant trait ('S'), while disliking requires two recessive alleles ('s').

Example Cross

• A heterozygous cat for hair presence and sink liking ('HhSs') is crossed with a hairless and sink-disliking cat ('hhss').
• 'Hh' indicates the first cat is heterozygous for the hair trait, and 'Ss' indicates it is heterozygous for the sink-liking trait.
• 'hh' means the second cat is homozygous recessive for the hair trait (no hair), and 'ss' means it is homozygous recessive for the sink-liking trait (dislikes sinks).

Parental Gametes and Punnett Squares

• Parental alleles are positioned on the corners of a Punnett square, representing alleles within gametes (sperm or egg).
• Gametes contain half the DNA of regular cells, carrying only one allele for each gene due to Mendel’s law of segregation.
• Mendel’s law of segregation states a gamete carries only one allele for each gene.
• Each gamete of a cat with 'HhSs' genotype carries two alleles for different traits due to Mendel's law of independent assortment.
• Mendel's law of independent assortment suggests one trait's inheritance is independent of another if the genes aren't linked.
• A cat's hair presence doesn't influence its sink preference because the genes are not linked.

Creating a Dihybrid Punnett Square

• List all possible gamete combinations from both parents on the 16-square Punnett square's top and side.
• The FOIL method (First, Outside, Inside, Last) is used to determine gamete combinations.
• For a 'HhSs' cat, the possible gamete combinations are HS, Hs, hS, and hs.
• The only possible gamete combination for an 'hhss' cat is hs, because it's homozygous recessive for both traits.

Offspring Genotypes and Phenotypes

• Combine gametes to predict offspring traits, maintaining consistent notation (e.g., 'H' before 'h', 'S' before 's').
• Genotype refers to the genetic makeup, while phenotype refers to observable traits.
• The genotypic ratio in the example cross is 1:1:1:1 for all possible genotypes.
• The phenotypic ratio mirrors the genotypic ratio in this example, which isn't always the case.
• The chance of a kitten resembling "Moo" (having hair and liking sinks) is 4/16 or 25%.
• There is a 4/16 (25%) distribution for each of the following trait combinations: haired/sink-liking, haired/sink-disliking, hairless/sink-liking, and hairless/sink-disliking.

Important Considerations

• Phenotypic and genotypic ratios may not always align and ratios can vary.
• A Punnett square is a predictive tool, forecasting the probabilities of offspring inheriting specific genotypes or phenotypes.