Heredity - Questions and Answers

Chapter Questions

Exercises

Key Concepts

Chapter Questions

1. If a trait A exists in 10% of a population of an asexually reproducing species and a trait B exists in 60% of the same population, which trait is likely to have arisen earlier?

Trait B is likely to have arisen earlier because in asexual reproduction, traits that appear earlier have more time to spread through the population through successive generations of reproduction.

2. How does the creation of variations in a species promote survival?

Variations in a species promote survival by providing diversity that allows some individuals to better adapt to changing environmental conditions. For example, bacteria with heat resistance would survive better during a heat wave. This selection of advantageous variants forms the basis for evolutionary processes.

3. How do Mendel's experiments show that traits may be dominant or recessive?

Mendel's experiments showed that when he crossed tall and short pea plants, all F1 progeny were tall. However, when these F1 plants were self-pollinated, the F2 generation had both tall and short plants in a 3:1 ratio. This indicates that the tall trait was dominant (expressed even when only one copy was present) and the short trait was recessive (expressed only when two copies were present).

4. How do Mendel's experiments show that traits are inherited independently?

Mendel demonstrated independent inheritance by studying two traits simultaneously, such as seed shape and seed color. When he crossed plants with round yellow seeds and wrinkled green seeds, the F1 generation all had round yellow seeds. But in the F2 generation, he observed new combinations like round green and wrinkled yellow seeds, showing that traits for seed shape and seed color were inherited independently of each other.

5. A man with blood group A marries a woman with blood group O and their daughter has blood group O. Is this information enough to tell you which of the traits - blood group A or O - is dominant? Why or why not?

No, this information alone is not enough to determine which blood group is dominant. Blood group inheritance follows ABO system where A and B are codominant and both are dominant over O. The man could have genotype AO (expressing blood group A) and the woman has OO (expressing blood group O). Their daughter could inherit O from both parents, resulting in blood group O. Without knowing the genotypes of the parents, we cannot conclusively determine dominance.

6. How is the sex of the child determined in human beings?

In human beings, sex is determined by the sex chromosomes. Women have two X chromosomes (XX), while men have one X and one Y chromosome (XY). All children inherit an X chromosome from their mother. The sex of the child is determined by the chromosome inherited from the father: if it's an X chromosome, the child will be female (XX); if it's a Y chromosome, the child will be male (XY).

Sex determination in human beings

Exercises

1. A Mendelian experiment consisted of breeding tall pea plants bearing violet flowers with short pea plants bearing white flowers. The progeny all bore violet flowers, but almost half of them were short. This suggests that the genetic make-up of the tall parent can be depicted as

The correct answer is (c) TtWw.

Explanation:

All progeny having violet flowers indicates that violet flower color is dominant over white.
The presence of short plants in the progeny (approximately half) indicates that the tall parent was heterozygous (Tt) for the height trait.
Since all flowers were violet, the parent must have been homozygous dominant (WW) for flower color.
Therefore, the genotype of the tall parent is TtWW.

2. A study found that children with light-coloured eyes are likely to have parents with light-coloured eyes. On this basis, can we say anything about whether the light eye colour trait is dominant or recessive? Why or why not?

No, we cannot definitively say whether light eye color is dominant or recessive based solely on this information.

If light eye color were dominant, children with light eyes would likely have at least one parent with light eyes. However, if light eye color were recessive, both parents would need to carry the recessive allele for the child to express light eyes. In this case, both parents would likely have light eyes themselves if they are homozygous recessive, or could have dark eyes if they are heterozygous carriers.

To determine dominance, we would need to examine inheritance patterns across multiple generations and consider cases where parents with different eye colors have children.

3. Outline a project which aims to find the dominant coat colour in dogs.

Project: Determining Dominant Coat Color in Dogs

Objective: To determine which coat color (black or brown) is dominant in a specific dog breed.
Materials: Purebred black and brown dogs of the same breed, records of parentage, camera for documentation.
Method:
- Select purebred black and brown dogs with known parentage.
- Cross a homozygous black dog with a homozygous brown dog.
- Observe and record the coat color of all F1 offspring.
- If all F1 offspring are black, then black is dominant; if all are brown, then brown is dominant.
- For confirmation, cross F1 offspring with each other and observe the F2 generation.
- Record the ratio of coat colors in F2 generation.
Expected Results: If black is dominant, F2 generation should show a 3:1 ratio of black to brown coats.
Conclusion: Based on the observations, determine which coat color is dominant and which is recessive.

4. How is the equal genetic contribution of male and female parents ensured in the progeny?

The equal genetic contribution of male and female parents is ensured through the process of sexual reproduction:

Each parent produces gametes (sperm and egg cells) through meiosis, which reduces the chromosome number by half.
During fertilization, one gamete from each parent combines to form a zygote.
The zygote thus receives half of its chromosomes from the male parent and half from the female parent.
Each chromosome carries genes, so the offspring inherits an equal number of genes from both parents.
This mechanism ensures that the genetic material is equally contributed by both parents, restoring the diploid chromosome number in the offspring.

Meiosis ensures equal genetic contribution from both parents

Key Concepts

Important Terms

Heredity: The passing of traits from parents to offspring.
Variation: Differences between individuals of the same species.
Gene: A section of DNA that provides information for one protein.
Allele: Different forms of the same gene.
Dominant trait: A trait that is expressed even when only one copy of the gene is present.
Recessive trait: A trait that is expressed only when two copies of the gene are present.
Genotype: The genetic makeup of an organism.
Phenotype: The observable characteristics of an organism.
Chromosome: A thread-like structure of DNA that carries genes.
Sex chromosomes: Chromosomes that determine the sex of an individual (X and Y in humans).

Mendel's Laws of Inheritance

Law of Dominance: In a pair of contrasting characters, one dominates the other.
Law of Segregation: During gamete formation, the alleles for each gene segregate so that each gamete carries only one allele for each gene.
Law of Independent Assortment: Genes for different traits are inherited independently of each other.

Key Points from the Chapter

Variations arising during reproduction can be inherited and may lead to increased survival.
Sexually reproducing individuals have two copies of genes for each trait.
If the gene copies are not identical, the dominant trait is expressed.
Traits can be inherited separately, creating new combinations in offspring.
In humans, sex is determined by whether the paternal chromosome is X (female) or Y (male).
DNA is the information source for making proteins that control traits.

Mendelian inheritance patterns