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Answers
Solution 11
Colour-blindness is caused due to recessive genes which occur on the X chromosome.
Males have only one X chromosome. If there is recessive gene present on X chromosome, then the male will suffer from colour-blindness.
Females have two X chromosomes. It is highly impossible that both the X chromosomes carry abnormal gene. Hence, if one gene is abnormal and since it is recessive, its expression will be masked by the normal gene present on the other X chromosome. Females are unlikely to suffer from colour-blindness.
Question 12
In a certain species of animals, black fur (B) is dominant over brown fur (b). Show the possible ratio of genotypes and phenotypes of the offspring of pure breeding different coloured parents.
Solution 12
Phenotypic Ratio - 3 (Black Fur) :1 (Brown Fur)
Genotypic Ratio - 1(Homozygous Black Fur):2 (Heterozygous Black Fur): 1 (Homozygous Brown Fur)
Question 13
Explain the following terms:
(a) Heterozygous
(b) Homozygous
(c) Pedigree chart
Solution 13
(a) Heterozygous: The condition in which a pair of homologous chromosomes carries dissimilar alleles for a particular character.
For example -
(i) A daughter (XXo) from a normal homozygous mother for colour vision (XX) and a colour blind father has one normal and one defective allele (XoY).
(ii) Certain tongue rollers are heterozygous with Rr genotype.
(b) Homozygous: The condition in which a pair of homologous chromosomes carries similar alleles for a particular character.
For example -
(i) A colorblind daughter (XoXo) will have both the X chromosomes with defective alleles.
(ii) A non-roller will have rr (homozygous) genotype.
(c) Pedigree Chart: A pedigree chart is a diagram that shows the occurrence and appearance or phenotypes of a particular gene or organism and its ancestors from one generation to the next. In the pedigree chart, males are shown by squares and females by circles.
Question 14
State the three Mendel's laws of inheritance.
Solution 14
Mendel's laws of inheritance are:
(i) Law of Dominance Out of a pair of contrasting characters present together, only one is able to express itself while the other remains suppressed. The one that expresses is the dominant character and the one that is unexpressed is the recessive one.
(ii) Law of Segregation : The two members of a pair of factors separate during the formation of gametes. The gametes combine together by random fusion at the time of zygote formation. This law is also known as 'law of purity of gametes'.
(iii) Law of Independent Assortment: When there are two pairs of contrasting characters, the distribution of the members of one pair into the gametes is independent of the distribution of the other pair.
Question 15
Does the sex of the child depend on the father or is it just a matter of chance? Discuss.
Solution 15
The sex of the child depends on the father. The egg contains only one X chromosome, but half of the sperms contain X-chromosome whereas the other half contains Y-chromosome. It is simply a matter of chance as to which category of sperm fuses with the ovum and this determines whether the child will be male or female.
If the egg fuses with X-bearing sperm, the resulting combination is XX and the resulting child is female.
If the egg fuses with Y-bearing sperm, the resulting combination is XY and the resulting child is male.
Question 16
In a certain species of animals, black fur (B) is dominant over brown fur (b). Predict the genotype and phenotype of the offspring, when both parents are 'Bb' or have heterozygous black fur.
Solution 16
B
b
B
BB
Bb
b
Bb
bb
Genotype - 1(Homozygous Black Fur) :2 (Heterozygous Black Fur):1 (Homozygous Brown Fur)
Phenotype - 3 (Black Fur) :1(Brown Fur)
Question 17
Two pairs (A and B) of rabbits were crossed as given below:
(a) Can you tell which coat colour (black or white) is dominant?
(b) Is the coat colour sex-linked?
Solution 17
(a) Black
(b) No
Question 18
Make a Punnett square for finding out the proportion of different genotypes in the progeny of a genetic cross between
(a) A pure tall (TT) pea plant with a pure dwarf (tt) pea plant.
(b) Red flower variety of pea (RR) with white flower variety of pea (rr).
Solution 18
(a)
F2 generation -
G
Genotype - 1(Homozygous tall) :2 (Heterozygous tall):1 (Homozygous dwarf)
Phenotype - 3 (Tall) :1(Dwarf)
(
Genotype - 1(Homozygous red) :2 (Heterozygous red):1 (Homozygous white)
Phenotype - 3 (Red) :1(White)
Question 19
A family consists of two parents and their five children and the pedigree chart shown below shows the inheritance of the trait colour blindness in them.
(a) Who is colour blind in the parents - the Father or the Mother?
(b) How many daughters and how many sons have been born in the family?
(c) What does the child 1 indicate about this trait?
(d) On which chromosome is the gene of this trait located?
(e) Name one other trait in humans which follows a similar pattern of inheritance.
Solution 19
(a) Father
(b) Two sons and three daughters
(c) The child 1 (daughter) is colour blind
(d) X chromosome
(e) Haemophilia
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