Heredity — Class 10 Science
Science · Chapter 8
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Chapter summary
Key ideas & terms
- Gene
- A unit of heredity that controls a particular trait.
- Dominant trait
- A trait expressed even when only one allele is present.
- Recessive trait
- A trait expressed only when both alleles are recessive.
- Sex determination
- How the sex of offspring is decided by sex chromosomes.
Extra questions & answers
How is the sex of a child determined in humans?
The mother contributes an X chromosome; if the father contributes X the child is a girl (XX), and if Y, a boy (XY).
Why did Mendel choose the pea plant for his experiments?
Pea plants have clear contrasting traits, a short life cycle and can be easily self- or cross-pollinated.
What is heredity?
The transmission of characters (traits) from parents to their offspring through genes carried on the chromosomes.
Why did Mendel choose the garden pea for his experiments?
The pea plant has several clear contrasting traits, a short life cycle, and can be easily self-pollinated or cross-pollinated.
Distinguish between dominant and recessive traits.
A dominant trait is expressed even when only one copy of its gene (allele) is present, while a recessive trait is expressed only when both alleles are recessive.
What ratios did Mendel obtain in a monohybrid and a dihybrid cross?
A 3:1 ratio in the F₂ generation of a monohybrid cross and a 9:3:3:1 ratio in a dihybrid cross.
What do Mendel’s dihybrid results show?
That two pairs of traits are inherited independently of each other (independent assortment of genes).
How is the sex of a child determined in humans?
The mother always contributes an X chromosome; if the father contributes an X the child is a girl (XX), and if a Y, the child is a boy (XY).
What is a gene?
A unit of heredity, a section of DNA on a chromosome, that controls a particular trait by directing the making of a specific protein.
Why are traits inherited through genes and not acquired traits?
Only changes in the genes of the reproductive cells are passed on; traits acquired during an individual’s lifetime do not change the genes and so are not inherited.
QUESTIONS
1. If a trait A exists in
Trait B is likely to have arisen earlier. In an asexually reproducing population, an older heritable variation generally has had more generations in which to be passed on, so it is likely to occur in a larger proportion of the population.
2. How does the creation of variations in a species promote survival?
Variations make individuals within a species different from one another. When environmental conditions change, some individuals may possess variations that help them survive and reproduce. These useful variations are then passed to later generations, helping the species continue.
Activity 8.1
Observe the ears of all the students in the class. Prepare a list of students having free or attached earlobes and calculate the percentage of students having each (Fig. 8.2). Find out about the earlobes of the parents of each student in the class. Correlate the earlobe type of each student with that of their parents. Based on this evidence, suggest a possible rule for the inheritance of earlobe types.
A class survey may show that some students have free earlobes while others have attached earlobes. The percentage of each type can be calculated as
Activity 8.2
In Fig. 8.3, what experiment would we do to confirm that the
Each
QUESTIONS
1. How do Mendel’s experiments show that traits may be dominant or recessive?
Mendel crossed pure tall pea plants with pure short pea plants. All the plants in the
2. How do Mendel’s experiments show that traits are inherited independently?
Mendel crossed pea plants differing in two traits and then self-pollinated the
3. 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?
Yes. The daughter with blood group O must have received an O allele from each parent. Therefore, the father with blood group A must carry both an A allele and an O allele, but he expresses blood group A. This shows that the A trait is dominant over the O trait, while O is recessive.
4. How is the sex of the child determined in human beings?
A mother has
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
(a)
(b)
(c)
(d)
The correct answer is (c)
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. The observation only shows that light eye colour can be inherited from parents. It does not provide information about crosses between people with contrasting eye colours or about how the trait behaves when different alleles occur together. Therefore, dominance or recessiveness cannot be determined from this study alone.
3. Outline a project which aims to find the dominant coat colour in dogs.
Select dogs with two contrasting coat colours and record the coat colours of the parents and their offspring over several families or generations. Begin with parents that consistently produce offspring of their own coat colour, and cross dogs of the two types. If all first-generation offspring show one colour, that colour is a possible dominant trait. Breed suitable first-generation dogs and examine the next generation. The reappearance of the other colour would support the conclusion that it is recessive. A large sample and careful pedigree records should be used to make the result reliable.
4. How is the equal genetic contribution of male and female parents ensured in the progeny?
During the formation of germ cells, each parent produces cells containing only one chromosome from every pair. At fertilisation, the male and female germ cells unite, and each contributes one complete set of chromosomes. The normal paired chromosome number is restored in the offspring, so the male and female parents contribute equal amounts of genetic material.