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Dihybrid crosses and gene linkage. Topic 10.2. Assessment Statements. 10.2.1 Calculate and predict the genotypic and phenotypic ratio of offspring of dihybrid crosses involving unlinked autosomal genes. 10.2.2 Distinguish between autosomes and sex chromosomes .

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assessment statements
Assessment Statements

10.2.1Calculate and predict the genotypic and phenotypic ratio of offspring of dihybrid crosses involving unlinked autosomal genes.

10.2.2Distinguish between autosomes and sex chromosomes.

10.2.3Explain how crossing over between non-sister chromatids of a homologous pair in prophase I can result in an exchange of alleles.

10.2.4Define linkage group.

10.2.5 Explain an example of a cross between two linked genes.

10.2.6 Identify which of the offspring are recombinants in a dihybrid cross involving linked genes.

dihybrid crosses
Dihybrid crosses
  • Observations:
    • Seed shape – some round, others wrinkled (allele for round is dominant)
    • Seed colour– some green, others yellow (allele for yellow is dominant)
  • Mendel crossed true breeding plants with each other
    • One parent: homozygous dominant for both traits (round and yellow seeds)
    • Other parent: homozygous recessive for both traits (wrinkled and green)

R = allele for round peas

  • r = allele for wrinkled peas
  • Y = allele for yellow peas
  • y = allele for green peas

Allowing heterozygous offspring to self-pollinate yields following proportions:

    • Round and yellow 56.6%
    • Wrinkled and yellow 18.2%
    • Round and green 19.4%
    • Wrinkled and green 8%
  • Ratio
    • 9:3:3:1
autosomes and sex chromosomes
Autosomes and sex chromosomes
  • Sex chromosomes: X and Y (one pair)
  • Autosomes:any chromosome not X or Y (22 pairs)
  • Sex-linked gene or trait is located on a sex chromosome.
  • Autosomal gene or trait is located on one of the autosomes.
  • On which type of chromosome is the gene for protein production in the testes found?
  • So, the gene is known as ___.
  • On which type of chromosome is the gene for protein production in the pancreas found?
  • So, the gene is known as ___.
exchange of alleles by crossing over
Exchange of alleles by crossing over

Two non-sister chromatids can swap segments of their DNA

A maternal chromosome can end up with a segment of a paternal chromosome and vice versa

linkage group
Linkage group

Any two genes which are found on the same chromosome are said to be linked to each other.

Linked genes are usually passed on to the next generation together.

Linkage group - groups of genes on the same chromosome inherited together

Linked genes are the exception to Mendel’s law of independent assortment

linked genes
Linked genes
  • Fruit fly gene for body color is in the same linkage group as the gene for wing length
  • G – grey body
  • g – black body
  • L – long wings
  • l – short wings
  • GGLL – gray body, long wings
  • ggll – black body, short wings
  • G L
  • G L
  • Two horizontal bars symbolize homologous chromosomes and that the locus of G is on the same chromosome as L
  • F2 generation genes located on same pair of chromosomes, so only possible gametes are PL and pl
  • Phenotypic ratio is 3:1
offspring of a dihybrid cross
Offspring of a dihybrid cross

A cross between homozygous dominant true-breeding fruit fly (GGLL) and a homozygous recessive true-breeding fruit fly (ggll) would result in flies which were all heterozygous for both of the traits (GgLl)

If heterozygotes for these traits are mixed with homozygous dominant flies, then one could not determine the genotype by sight.

What to do?

test cross
Test Cross
  • A new shuffling of the alleles has created a new combination which does not match either of the parents’ genotypes
  • The term recombinant is used to describe both the new chromosome and the resulting organism.
  • Recombinants form through the process of crossing over
  • Cross “mystery” fly with homozygous recessive (ggll)
  • Assuming the mystery fly is heterozygous (GgLl) for both traits:
assessment statements1
Assessment Statements

10.3.1 Define polygenic inheritance.

10.3.2 Explain that polygenic inheritance can contribute to continuous variation using two examples, one of which must be human skin colour.


Involves two or more genes influencing the expression of one trait

Increased number of possible genotypes

Believed that most human traits are too complex to be determined by one gene

continuous and discontinuous variation
Continuous and discontinuous variation
  • When an array of possible phenotypes can be produced, it is called continuous variation
    • Examples: skin color, height, body shape, and intelligence
    • These traits are also influenced by environmental conditions
  • When only a number of phenotypes can be produced, it is called discontinuous variation
    • Examples: earlobe attachment, widow’s peak, blood type
graphical representation
Graphical representation

Continuous variation

Discontinuous variation

eye color
Eye Color
  • Iris is made up of zones, rings, streaks or speckles of different colored pigments with varying intensities
  • What color are your eyes, really?
  • Since there is so much variety, eye color must be influenced by multiple alleles and has continuous variation.
skin color
Skin color
  • Many societies feel the need to label people with categories
  • Oversimplification serves more administrative purposes than any biological purpose
  • So what is the purpose of skin?!
purpose of melanin skin pigment
Purpose of melanin (skin pigment)
  • Melanin protects from serious sunburn which can lead to melanoma
  • Melanin is found in all individuals except albinos
  • Concentration of melanin varies
  • How can one increase the melanin level in their skin?
  • Tanning is a natural defense against the negative effects of excess sunlight
uv light and calciferol vitamin d
UV light and calciferol (vitamin D)

UVB radiation helps the skin to make vitamin D

Vitamin D is necessary for proper growth and bone formation

It is good to have a MODERATE amount of sunlight

In regions of low Sun exposure people need light-colored skin


  • How do people of varying degrees of skin color relocated to parts of the world that receive differing amounts of sunlight get vitamin D? How do others fight off the sun?
  • Should there be equal esteem for all humans?
  • Why is human diversity so often used to divide and discriminate, rather than be appreciated, respected, and celebrated?