D3.2 Analysis of Inheritance Patterns [IB Biology SL/HL]

Summary

This video explores the patterns of inheritance, with a focus on sex-linked genetic disorders like hemophilia. It covers how sex-linked genes, such as those found on the X chromosome, are inherited differently in males and females. Males cannot be carriers of hemophilia, whereas females can be carriers without showing symptoms. The video also explains how to use Punnett squares to predict inheritance outcomes, and how pedigree charts help track genetic conditions across generations. It distinguishes between discrete and continuous variation in traits, with practical examples such as blood type and skin color.

Takeaways

• 🧬 Hemophilia is an X-linked recessive disorder causing clotting factor deficiency, leading to excessive bleeding.
• 👩 Females can be carriers of hemophilia if heterozygous, but only homozygous recessive females are affected.
• 👨 Males cannot be carriers; if their X chromosome carries the recessive allele, they will have hemophilia.
• 📊 Punnett squares can predict offspring outcomes for sex-linked traits, including hemophilia, with phenotypic ratios like 3 normal : 1 affected.
• 👪 Pedigree charts use circles for females, squares for males, and shading indicates affected individuals.
• 🔍 In pedigree analysis, homozygous recessive individuals show autosomal recessive diseases, while unaffected parents may be carriers.
• ⚠️ Genetic disorders are rare but can be more common with inbreeding, increasing the chance that both parents carry the same recessive allele.
• 🎨 Discrete variation arises from single-gene traits, producing clear categories like blood type or hemophilia.
• 🌈 Continuous variation results from polygenic traits influenced by multiple genes and environmental factors, producing a range of phenotypes.
• 📈 Box-and-whisker plots display continuous data, showing medians, quartiles, and interquartile ranges to identify patterns and outliers.
• 🧮 Outliers are mathematically determined using 1.5 × IQR added to Q3 or subtracted from Q1, not just visually or subjectively.

Q & A

What is hemophilia and how is it inherited?

Hemophilia is a sex-linked recessive disorder caused by a gene on the X chromosome. It results in a deficiency in clotting factors, leading to excessive bleeding. Females can be carriers or affected if homozygous recessive, while males with the recessive allele on their single X chromosome are affected.

Why can males not be carriers of X-linked recessive disorders?

Males cannot be carriers because they have only one X chromosome. If their single X chromosome carries the recessive allele, they will express the disorder rather than carry it asymptomatically.

How do you represent males and females in a pedigree chart?

In pedigree charts, circles represent females and squares represent males. Shaded symbols indicate individuals affected by a genetic disorder, while unshaded symbols indicate unaffected individuals.

What is the difference between discrete and continuous variation?

Discrete variation refers to traits controlled by a single gene with distinct categories (e.g., blood type), while continuous variation involves traits influenced by multiple genes and environmental factors, resulting in a spectrum of phenotypes (e.g., height, skin color).

How do you determine outliers using a box-and-whisker plot?

To determine outliers: calculate the interquartile range (IQR = Q3 − Q1), multiply IQR by 1.5, then subtract this value from Q1 and add it to Q3. Values outside this range are considered outliers.

What is the phenotypic ratio when a male with normal clotting mates with a carrier female for hemophilia?

The phenotypic ratio is 3:1, with three individuals showing normal blood clotting (including one carrier female) and one individual affected by hemophilia. Among male offspring, 50% are expected to have hemophilia.

How can you deduce genotypes in a pedigree chart for an autosomal recessive disorder?

Start by identifying individuals with the disease, as they must be homozygous recessive. Then trace alleles from parents to determine carriers, ensuring that assumptions are based on observed inheritance patterns rather than speculation.

Why are recessive genetic disorders more common in inbreeding?

Recessive alleles are usually rare in the general population. In inbreeding, close relatives share the same recessive alleles, increasing the probability that offspring inherit two copies and express the disorder.

What are carriers and how do they function in inheritance?

Carriers are individuals who possess one copy of a recessive allele but do not show symptoms of the disorder because it is masked by a dominant allele. Only females can be carriers for X-linked recessive disorders.

What is the importance of using proper notation in Punnett squares?

Using proper notation ensures accurate representation of gametes and offspring genotypes. It allows for precise calculation of probabilities for inheriting specific traits and avoids confusion in genetic analysis.

How does environmental influence affect continuous traits?

Environmental factors can modify the expression of polygenic traits, creating a range of phenotypes rather than discrete categories. For example, nutrition, sunlight exposure, and lifestyle can influence height and skin color.