SBI3U1 - Sex Linkage and Pedigrees - SLIDES (1)

Sex-Linked Inheritance and Pedigrees

Sex Linkage

• Chromosome Pairs: Humans possess 23 pairs of chromosomes, consisting of 22 pairs of autosomes and 1 pair of sex chromosomes (XX in females and XY in males). This genetic framework plays a critical role in determining sexual characteristics and hereditary traits.

• Gene Location: Genes situated on sex chromosomes exhibit unique inheritance patterns due to the differentiation between X and Y chromosomes, with the X chromosome harboring many more genes than the Y chromosome. The presence of specific alleles on these chromosomes leads to varied phenotypic expression dependent on the sex of the individual.

X-Linked and Y-Linked Traits

• X-Linked Genes: Genes located on the X-chromosome often exhibit different inheritance patterns in males and females. Since males have only one X chromosome, any recessive mutation present on it will be expressed, a phenomenon not typically seen in females, who possess two X chromosomes.

• Y-Linked Genes: Genes found on the Y-chromosome are less prevalent due to the smaller size of the Y chromosome, resulting in fewer genes. Traits and disorders that are Y-linked are exclusively inherited from father to son.

• Y-Linked Disorders: The rarity of Y-linked disorders stems from the limited number of genes on the Y chromosome; disorders can impact male fertility and sex determination.

Examples of X-Linked Recessive Traits:

• Red-green color blindness: A common form of color blindness where the ability to distinguish between red and green colors is impaired due to a defect in photoreceptor cells.

• Male-pattern baldness: An inherited condition that leads to hair loss in specific patterns, more prevalent in males due to its X-linked recessive nature.

• Hemophilia: A genetic disorder causing excessive bleeding due to the absence or deficiency of specific clotting factors, predominantly affecting males.

Inheritance in Males:

Males, having only one copy of the X chromosome, are more likely to express X-linked recessive traits due to the hemizygous situation. Females, on the other hand, may act as carriers if they possess one normal allele and one affected allele, thereby potentially passing the trait to their offspring.

Punnett Square:

• Utility: A tool used to calculate the probability of traits in offspring based on parental alleles. It visually represents the possible combinations of alleles from the parents.

Sample Problem on Colorblindness

• Trait: Colorblindness is an X-linked recessive trait.

• Parental Genotypes:

  • Father: XbY (colorblind)

  • Mother: XBXb (heterozygous normal vision)

• Probability Calculation:

  • There is a 50% chance of them having a colorblind child based on their genotypes.

• Offspring Phenotypes:

  • 25% colorblind female (XbXb)

  • 25% normal vision female (XBXb)

  • 25% colorblind male (XbY)

  • 25% normal vision male (XBY)

Pedigrees

• Definition: Pedigrees are graphical representations that depict the inheritance of traits across multiple generations, allowing for the tracing of genetic conditions within a family tree format.

• Components: They illustrate relationships among individuals (parents, siblings) and denote the presence or absence of specific traits along with the biological sex of each individual.

• Notations for affected and normal individuals:

  • Normal male (□)

  • Affected male (⌬)

  • Carrier female (●)

  • Normal female (◻)

  • Siblings (horizontal lines connecting)

  • Identical twins (joined shapes, usually vertical line)

  • Fraternal twins (separate shapes)

  • Deceased individuals (diagonal line through the shape)

Reading a Pedigree

• Identifying Affected Individuals: Affected individuals in the family can be noted by specific symbols, such as I-1, II-3, II-4, III-3, which denote the presence of the trait.

• Count of Individuals: Total: 6 females and 4 males in the pedigree.

• Generational Analysis: This includes analyzing the number of children in the second generation: 4, and noting that the oldest child in the third generation does not exhibit the trait.

Inheritance Patterns

• Analysis Using Pedigrees: Evaluate the mode of inheritance of traits/disorders based on:

  • Dominance: Is the trait associated with a dominant or recessive allele?

  • Chromosomal Location: Is the trait determined by an autosome or a sex chromosome?

Inheritance Patterns Summary

Pedigree Analysis: Sample Problem 1

• Case Study: Marfan syndrome affects connective tissue; dominant allele (M) indicates syndrome presence.

• Genotype Analysis: Individuals expressing the syndrome will have at least one M allele, while unaffected individuals will be genotype mm (homozygous recessive).

Determining Unknown Genotypes

• Method: Infer unknown genotypes by analyzing offspring-parent relationships and known alleles within the pedigree.

Pedigree Analysis: Sample Problem 2

• Condition: Albinism results from a defective enzyme affecting melanin production, governed by a normal allele (P) and an albinism allele (p).

• Analysis Requirements: Determine whether albinism is dominant or recessive based on pedigree clues and establish individual genotypes using algebraic symbols for dominance.

Summary on Albinism Inheritance

• Trait Characterization: Since affected individuals lack affected parents, the trait is recessively inherited and does not follow X-linked dominance.

• Genotyping Individuals: Affected individuals must be pp, while others must possess at least one dominant allele (P).

Pedigree Analysis: Sample Problem 3

• Family Case Study: George (with albinism) marries Mary (unaffected). They have a daughter, Frances (albinism), who marries Jack (whose family includes albinism). Their daughter Anne shows no symptoms.

• Objective: Construct a family pedigree and determine each individual's genotype.

• Genotypes of the Given Family:

  • George (albinism): aa

  • Mary (normal): Aa

  • Frances (albinism): aa

  • Jack (albinism): aa

  • Anne (normal): Aa.