Question
▶️Answer/Explanat
Question
The pedigree chart for a family shows the inheritance of Huntington’s disease.
▶️Answer/Explanation
Question
A farmer was interested in the yield produced by his crop. The graph displays the number of seeds of different masses in the crop yield. Which statement could clarify the inheritance of this trait?
A. Inheritance of seed mass follows Mendel’s laws for a monohybrid cross.
B. This is an example of discontinuous variation.
C. Two linked genes are responsible for seed mass in this plant.
D. Polygenic inheritance results in subtle phenotypic differences.
▶️Answer/Explanation
Ans. D
Polygenic inheritance refers to the type of genetic inheritance where multiple genes, often located at different loci, contribute to the expression of a single trait. Unlike Mendelian inheritance, which involves one or a few genes with clear dominant and recessive alleles, polygenic traits are influenced by the combined effects of many genes. This results in a continuous range of phenotypic variation, rather than distinct categories.
As a result, polygenic inheritance typically leads to subtle phenotypic differences within a population. For example, traits such as human skin color, height, and intelligence are all influenced by multiple genes. Because each of these genes contributes a small effect, the overall outcome is a smooth gradation of characteristics rather than distinct, easily identifiable groups. This continuous variation is often described as a bell curve, with most individuals exhibiting intermediate traits and fewer individuals showing extreme phenotypes at either end.
In summary, polygenic inheritance produces traits that show a wide spectrum of phenotypic variation, making the differences between individuals more gradual and less noticeable than those caused by single-gene (Mendelian) inheritance.
Question
Non-syndromic hearing loss and deafness (DFNB1) is an inherited form of deafness in humans. The pedigree chart illustrates the transmission of DFNB1 within a family.
Where is the DFNB1 allele found in family members with this condition?
A. On the Y chromosome
B. On the X chromosome
C. On one autosome only
D. On a pair of autosomes
▶️Answer/Explanation
Ans. D
DETERMINING THE MODE OF INHERITANCE
- Autosomal Dominant:
- The disease appears in every generation.
- An affected person has at least one affected parent.
- Both males and females are equally affected, and they can pass the trait to offspring.
- Affected individuals typically have an affected parent, and the disease often appears in both sexes equally.
- Autosomal Recessive:
- The disease may skip generations.
- Affected individuals can have unaffected parents who are carriers.
- Both males and females are equally affected.
- Two carrier parents (heterozygous) can have an affected child (homozygous recessive).
- X-Linked Dominant:
- The disease is more commonly seen in females than in males (because females have two X chromosomes, whereas males only have one).
- An affected father will pass the trait to all his daughters (since daughters inherit his X chromosome) but none of his sons (since sons inherit his Y chromosome).
- X-Linked Recessive:
- Males are more frequently affected because they have only one X chromosome.
- Affected males pass the trait to all their daughters (who will be carriers) but not to their sons.
- Carrier females (heterozygous) can pass the trait to sons, who will be affected.
- Y-Linked (Holandric) Inheritance:
- The disease only affects males, as it is carried on the Y chromosome.
- The trait is passed directly from father to son, and no females are affected.
Look for the Generation Patterns
- Autosomal Dominant:
- The disease appears in every generation, with affected individuals typically having one affected parent.
- Autosomal Recessive:
- The disease may skip generations (i.e., unaffected parents can have affected children).
- X-Linked Dominant:
- Affected fathers will pass the condition to all their daughters (but not to their sons).
- X-Linked Recessive:
- More males are affected than females. Affected males transmit the condition to all their daughters but not to their sons.
Examine the Parental Relationships and Offspring
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Consanguinity (Inbreeding): If the pedigree includes consanguineous relationships (marriage between relatives), the inheritance pattern might suggest a recessive disorder, as inbreeding increases the likelihood of recessive genes being inherited.
-
Carrier Detection: If both parents are unaffected but have an affected child, the parents are likely carriers of a recessive disease (for autosomal recessive or X-linked recessive inheritance).
Look for Clusters in Specific Generations
- Multiple Affected Individuals in a Single Generation: This can indicate an autosomal dominant pattern, especially if the disease appears in every generation.
- Disease Present Only in One Sex: This could suggest an X-linked or Y-linked pattern, with X-linked recessive diseases typically affecting males more frequently.
Question
Black, short-haired guinea pigs, heterozygous for both characteristics, were crossed. They produced offspring with the phenotypes black short-haired, black long-haired, white short-haired and white long-haired in the ratio 9:3:3:1. A different cross produced offspring with phenotypes in the ratio $1: 1: 1: 1$. What were the genotypes of the parents in the second cross?
A. $\mathrm{BbSs} \times \mathrm{BbSs}$
B. $\mathrm{BBSs} \times \mathrm{BbSS}$
C. $\mathrm{BbSs} \times$ bbss
D. $\mathrm{bbSS} \times \mathrm{BBsS}$
Answer/Explanation
Ans:C
Question
An individual is heterozygous for two linked genes \(\frac{AB}{ab}\)
To investigate the frequency of crossing over, a test cross is carried out between the individual and
another that is homozygous recessive for both genes. What are the possible recombinants in the
offspring of this cross?
Answer/Explanation
Ans: C