How is the sex of a newborn determined in humans?
The sex of the individual is genetically determined. Genes inherited from parents decide whether the newborn will be a boy or a girl. A newborn who inherits an 'X' chromosome from the father will be a girl, and one who inherits a 'Y' chromosome from the father will be a boy.
Do genetic combination of mothers play a significant role in determining the sex of a newborn?
No. Mothers have a pair of X chromosomes, so all children inherit an 'X' chromosome from their mother regardless of whether they are boys or girls. The sex of the child is determined by the type of sex chromosome contributed by the father (X or Y).
Mention three important features of fossils which help in the study of evolution.
Fossils represent modes of preservation of ancient species.
Fossils help in establishing evolutionary traits among organisms and their ancestors.
Fossils help in establishing the time period in which organisms lived.
Why do all the gametes formed in human females have an X chromosome?
Human females have two X chromosomes (XX). During meiosis at the time of gamete formation, one X chromosome enters each gamete. Hence all the gametes produced by a female contain an X chromosome.
In human beings, the statistical probability of getting either a male or female child is 50 : 50. Give a suitable explanation.
The sex of an infant is determined by the type of sex chromosome contributed by the male gamete. Male gametes are of two types — those containing an X chromosome and those containing a Y chromosome. Since the ratio of X-bearing to Y-bearing sperm is approximately 1:1, the statistical probability of having a male or female child is 50 : 50.
A very small population of a species faces a greater threat of extinction than a larger population. Provide a suitable genetic explanation.
Smaller populations often undergo extensive inbreeding which reduces genetic variation. Limited variation reduces the ability of the population to adapt to environmental changes, putting the species at greater risk of extinction if conditions change.
What are homologous structures? Give an example. Is it necessary that homologous structures always have a common ancestor?
Homologous structures are structures that have a common basic plan but perform different functions in different organisms. For example, the forelimbs of reptiles, amphibians and mammals are homologous — they share a common basic structure modified for different activities.
Yes, homologous structures usually indicate a common ancestor from which the structures were modified, though the modifications suit different functions.
Does the occurrence of diversity of animals on earth suggest their diverse ancestry also? Discuss this point in the light of evolution.
Although animals show vast diversity, this does not necessarily imply completely diverse ancestry for each group. A common ancestry in all cases may limit the extent of diversity. Many diverse animals inhabit similar habitats and their evolution by geographical isolation and speciation may not be likely in every case. Thus a single common ancestor for all animals is not the most likely theory; diversity can arise from branching evolution and varied selective pressures.
Give the pair of contrasting traits of the following characters in pea plant and mention which is dominant and recessive:
(i) yellow seed
(ii) round seed
(i) Yellow — dominant; green — recessive.
(ii) Round — dominant; wrinkled — recessive.
Why did Mendel choose pea plant for his experiments?
Mendel chose the pea plant because it was easy to grow, had a short life span, showed easily distinguishable characters, had sufficiently large flowers for manipulation, and was capable of self-pollination which allowed controlled crosses.
A woman has only daughters. Analyse the situation genetically and provide a suitable explanation.
(a) The woman produces ova that carry only the X chromosome.
(b) The man produces sperms carrying either X or Y chromosomes; it is the chromosome contributed by the sperm that actually determines the sex of the baby. If, in a series of children, the fertilizing sperms happened to carry X chromosomes each time, all children will be daughters.