Dihybrid crosses

Dihybrid Crosses

  • Definition: Dihybrid crosses are genetic crosses examining the inheritance pattern of two different traits/genes.

  • Genetic Principle: Genes are located on different chromosomes, leading to independent inheritance of these genes. This is exemplified by the Law of Independent Assortment.

  • Possible Outcomes:

    • In a dihybrid cross, there are 16 possible genotypes resulting from the combinations of alleles.

    • There are 4 different phenotypes that can arise.

Example 1: Guinea Pig Traits

  • Context: Male and female guinea pigs both heterozygous for fur color and texture.

  • Traits:

    • Dark fur (D) is dominant over light fur (d).

    • Rough fur (R) is dominant over smooth fur (r).

Parent Genotypes

  • Male Parent: Dd Rr

  • Female Parent: Dd Rr

Parent Phenotypes

  • Both parents exhibit:

    • Male: Dark and rough fur.

    • Female: Dark and rough fur.

Possible Gametes Produced by Each Parent

  • Male Parent (Dd Rr):

    1. DR

    2. Dr

    3. dR

    4. dr

  • Female Parent (Dd Rr):

    1. DR

    2. Dr

    3. dR

    4. dr

Number of Different Gamete Combinations

  • Each parent produces 4 distinct gametes.

Dihybrid Cross Construction

  • Gametes Used: DR, Dr, dR, dr

  • Offspring Genotypes:

    • DDRR

    • DDRr

    • DdRR

    • DdRr

    • DdRr

    • DDrr

    • DdRr

    • Ddrr

    • ddRR

    • ddRr

    • DdRr

    • Ddrr

    • ddrr

Frequency of Offspring that are Homozygous for Both Traits

  • Total outcomes for homozygous traits: 4 out of 16.

  • This results in a 25% frequency of homozygous offspring.

Frequency of Offspring with Dark, Rough Fur

  • Total outcomes indicating dark, rough fur: 9 out of 16.

Frequency of Offspring Expressing Both Recessive Traits

  • Total outcomes with both recessive traits: 1 out of 16.

Phenotypic Ratio of Offspring

  • Ratio Calculation:

    • Ratio for dark and rough : dark and smooth : light and rough : light and smooth is: 9 : 3 : 3 : 1.

Example 2: Cross of Guinea Pigs

  • Context: Male homozygous for dark fur and heterozygous for rough fur, crossed with a female heterozygous for dark fur and homozygous for smooth fur.

  • Traits: Same as previous example. (D = dark, d = light; R = rough, r = smooth)

Parent Genotypes

  • Male Parent: DDRr

  • Female Parent: Ddrr

Parent Phenotypes

  • Male: Dark and rough fur.

  • Female: Dark and smooth fur.

Possible Gametes Produced by Each Parent

  • Male Parent (DDrr):

    1. DR

    2. Dr

  • Female Parent (Ddrr):

    1. Dr

    2. dr

Number of Different Gamete Combinations

  • Each parent produces 2 distinct gametes.

Dihybrid Cross Construction

  • Gametes Used: DR, Dr, Dr, dr

  • Offspring Genotypes:

    • DDRR

    • DDRr

    • DdRr

    • Ddrr

Frequency of Offspring that are Homozygous for Both Traits

  • Total outcomes indicating homozygous traits: 4 out of 16.

  • This results in a 25% frequency of homozygous offspring.

Frequency of Offspring with Dark, Rough Fur

  • Total outcomes indicating dark, rough fur: 8 out of 16.

  • This results in a 50% frequency of dark, rough fur in offspring.

Dominance of Recessive Traits

  • Offspring expressing both recessive traits: 0 out of 16 (0%).

Phenotypic Ratio of Offspring

  • Ratio Calculation:

    • Ratio for dark and rough : dark and smooth : light and rough : light and smooth is: 1 : 1 : 0 : 0.

Mendel’s Examples Using Peas

  • Traits Considered: Round seeds (R) dominant over wrinkled seeds (r); Yellow color (Y) dominant over green color (y).

  • Parental Genotype Example: Yellow round x Yellow round

Gamete Production

  • Possible Gametes from both parents include: YR, Yr, yR, yr

Genetic Cross Outcomes

  • Outcomes yield various combinations like:

    • RR YY

    • RR Yy

    • Rr Yy

    • Rr YY

    • And others…

Genetic Ratios Observed

  • Ratios identified in offspring phenotype outcomes show a classic Mendelian ratio of 9:3:3:1.

Chromosome Theory of Inheritance

  • Historical Context: Proposed by Walter Sutton in 1902, linking Mendel’s findings.

  • Core Concept: Traits determined by genes on chromosomes are inherited through the movement of chromosomes during meiosis, critical for understanding the segregation and independent assortment of alleles.