Lecture 11 - Single Gene Inheritance

Question 1

What is Mendel’s model of inheritance?

Answer 1

Each parent has two alleles for a trait that segregate during gamete formation (Law of Segregation).
Gametes carry only one allele for each trait.
Offspring inherit one allele from each parent, determining the phenotype.

Question 2

What is a test for single-gene inheritance?

Answer 2

A 3:1 phenotypic ratio in F2 (genotype ratio 1:2:1) indicates single-gene inheritance.

Question 3

How does a test cross determine an unknown genotype?

Answer 3

Cross with a homozygous recessive individual:

Heterozygous: 50% dominant, 50% recessive offspring.

Homozygous dominant: All offspring show dominant phenotype.

Question 4

What are the four modes of single-gene inheritance?

Answer 4

Autosomal dominant.
Autosomal recessive.
Sex-linked dominant.
Sex-linked recessive.

Question 5

What is the difference between diploid and haploid organisms?

Answer 5

Diploid: Two copies of each chromosome (e.g., somatic cells).
Haploid: One copy of each chromosome (e.g., gametes).

Question 6

What are examples of diploid model organisms in genetics?

Answer 6

Saccharomyces cerevisiae (yeast).
Drosophila melanogaster (fruit fly).
Mus musculus (mouse).

Question 7

What is sex-linked inheritance?

Answer 7

Traits determined by genes on the X or Y chromosome.
Example: Eye colour gene in Drosophila is X-linked.

Question 8

How do X-linked recessive traits differ in males and females?

Answer 8

Males (XY): A single recessive allele results in the phenotype.

Females (XX): Both alleles must be recessive to show the phenotype.

Question 9

What is a pedigree?

Answer 9

A family tree diagram showing inheritance of traits or disorders.

Helps identify patterns like autosomal or sex-linked inheritance.

Question 10

How do you interpret a human pedigree?

Answer 10

Use symbols:
Squares for males, circles for females.
Shaded symbols indicate affected individuals.
Half-shaded indicate carriers (recessive traits).

Question 11

What is autosomal dominant inheritance?

Answer 11

Affects both sexes equally.
Appears in every generation.
Only one copy of the dominant allele is needed to show the trait.

Question 12

What is an example of an autosomal dominant disorder?

Answer 12

Marfan syndrome: Affects connective tissue, caused by mutations in the fibrillin gene on chromosome 15.

Question 13

What is autosomal recessive inheritance?

Answer 13

Affects siblings, both sexes equally.
Requires two copies of the recessive allele to show the trait.
Often seen in consanguineous matings.

Question 14

What is an example of an autosomal recessive disorder?

Answer 14

Phenylketonuria (PKU): Caused by a defect in the phenylalanine hydroxylase gene on chromosome 12.

Question 15

What is X-linked dominant inheritance?

Answer 15

Affected fathers pass the trait to all daughters but not sons.
Affected mothers pass the trait to both sons and daughters.

Question 16

What is X-linked recessive inheritance?

Answer 16

Affects mainly males.
Females are typically carriers unless homozygous.
Example: Haemophilia A.

Question 17

What is a Punnett square?

Answer 17

A diagram to predict genotypes and phenotypes of offspring from parental crosses.

Question 18

How are genes linked/unlinked written?

Answer 18

Linked genes: Written with a slash (e.g., AB/ab).

Unlinked genes: Written with a space or semicolon (e.g., A/a ; B/b).

Question 19

What does a 1:1 phenotypic ratio in a test cross indicate?

Answer 19

The F1 individual is heterozygous.

Question 20

What is the Law of Segregation?

Answer 20

Each parent contributes one allele for a trait, and alleles separate during gamete formation, ensuring offspring inherit one allele from each parent.

Question 21

What is the significance of a 3:1 phenotypic ratio in F2 generation?

Answer 21

It suggests inheritance of a single gene with one dominant and one recessive allele.

Question 22

What does the F2 phenotypic ratio indicate for X-linked recessive traits?

Answer 22

Male offspring are more likely to exhibit the recessive phenotype since they inherit a single X chromosome from the mother.

Question 23

How can Mendelian inheritance be observed in haploid organisms?

Answer 23

Haploid organisms (e.g., yeast) have only one allele per gene, so phenotypes directly reflect the genotype without dominance effects.

Question 24

What are key differences between autosomal and sex-linked inheritance?

Answer 24

Autosomal: Traits appear in both sexes equally.
Sex-linked: Traits often show sex-specific patterns (e.g., X-linked recessive traits are more common in males).

Question 25

What is the inheritance pattern in consanguineous matings?

Answer 25

Autosomal recessive traits are more likely to appear due to increased chances of inheriting the same recessive allele from both parents.

Question 26

How does X-linked inheritance explain male and female phenotypic differences?

Answer 26

Males (XY) are hemizygous for X-linked genes, so they express any allele present.
Females (XX) can be carriers or express the phenotype if homozygous.

Question 27

How do you determine the genotype of linked genes in a test cross?

Answer 27

Analyze offspring ratios for linked (e.g., AB/ab) or unlinked genes (e.g., A/a ; B/b).
Linked genes tend to show fewer recombinant phenotypes.

Question 28

Why do autosomal dominant traits appear in every generation?

Answer 28

Only one copy of the dominant allele is needed to express the trait, and affected individuals often pass it to offspring.

Question 29

Why is pedigree analysis important in human genetics?

Answer 29

Helps identify inheritance patterns for single-gene traits and diagnose genetic disorders.

Question 30

What are the key features of autosomal recessive pedigrees?

Answer 30

Often skips generations.
Affects both sexes equally.
Parents are usually carriers (heterozygous).

Question 31

What is the purpose of calculating probabilities in genetics?

Answer 31

To predict the likelihood of inheriting specific genotypes and phenotypes.
Example: Tossing a coin analogy for allele segregation.

Question 32

How do you differentiate between linked and unlinked genes in a genotype? (in writing)

Answer 32

Linked genes are separated by a slash (AB/ab).
Unlinked genes are separated by a semicolon or space (A/a ; B/b).