Lesson 2 Gen BIo

• Ishihara chart

  Used as a test for color blindness
• Sex Linkage and Recombination

  Explain sex linkage and recombination
• Describe how sex-linked traits are inherited


  1. Calculate the probability of the occurrence of a sex-linked trait

  2. Give examples of other sex-related traits
• Recombination

  A process by which pieces of DNA are broken and recombined to produce new combinations of alleles, creating genetic diversity
• Replication produces another copy of chromosomes
• Recombination increases genetic diversity. This is the reason why you and your sibling look different even if your genes are inherited from the same parents.
• Human Chromosomes

  • The 1st to 22nd pairs are called the autosomes, and the 23rd pair is termed as the sex chromosome
  • Genes that go along with either sex chromosome are said to be sex-linked
• 1. linked trait

  The X-linked trait is more common in males than in females
• Even if the mother is just a carrier of the trait and the father is normal, there is still a possibility that they will have an offspring with an X-linked trait.
• Generally, males have a 1/2 or 50% chance to express the X-linked trait. Females only have 1/3 or 33.3% chance of acquiring the trait.
• Color Blindness

  • Colorblindness is the inability to distinguish certain colors
  • The Ishihara chart is used as a test for color blindness
  • Eight Ishihara charts are available for testing color blindness
• Hemophilia

  • Hemophilia is a rare inherited bleeding disorder
  • People with this condition bleed longer after an injury as compared to a normal person
• 1. linked Trait

  • The Y-linked trait is only common in males since only males have the Y chromosome
  • An example is the hypertrichosis pinnae auris trait, which is characterized by having a hairy ear
• Possible color blindness genotypes and phenotypes of males and females

  • Female Genotypes: XX Normal female, X^C X^C Carrier female, X^C X^C Color-blind female

  Male Genotypes: X^C Y Color-blind male, XY Normal male
• Sex-Related Inheritance

  As opposed to Mendelian inheritance wherein the heterozygous genotype expresses the dominant trait, the heterozygous genotype of an X-linked trait in females will result in a carrier female
• The possible trait of the female offspring is 50% normal, 50% carrier. If the offspring is male, the chances are 50% hemophiliac and 50% normal.
• If Marie is color-blind and John has normal vision, they cannot have a daughter who is color-blind.
• Based on the Punnett square, the phenotype of the possible offspring if John is the father is carrier females and color-blind males.
• Sex-influenced Trait

  • Sex-influenced traits are controlled by autosomal genes, where the genes are found on both sexes, but one expresses it more than the other
  • Human pattern baldness is a sex-influenced characteristic
• Possible baldness genotypes and phenotypes of males and females

  • Female Genotypes: BB Non-bald, Bb Non-bald, bb Non-bald

  Male Genotypes: BB Bald, Bb Bald, bb Non-bald
• Sex-limited Trait

  • Sex-limited traits are also controlled by autosomal genes, where the genes are also found on both sexes, but only one sex expresses it
  • Lactation is a female-limited trait
• Possible lactation trait genotypes and phenotypes of males and females


  • Female Genotypes: RR Lactating, Rr Lactating, rr Not lactating

  Male Genotypes: RR Not lactating, Rr Not lactating, rr Not lactating
• Recombination

  A mechanism where the homologous pairs of chromosomes exchange genes
• Sex Linkage


  Termed as such because their phenotypic manifestation and inheritance patterns vary between males and females. Genes that go along with either sex chromosome are said to be sex-linked.

  1. linked trait: Linked to the X chromosome
  2. linked trait: Linked to the Y chromosome

  As opposed to Mendelian inheritance, the heterozygous genotype of an X-linked trait in females will result in a carrier female.
• Other sex-related traits are the sex-influenced and sex-limited traits.

  Sex-influenced traits can be found in both sexes but expressed more in one sex than in the other.

  Sex-limited traits have genes that can be found in both sexes but only one sex expresses it on their phenotype.