Samacheer Class 10 Science - Genetics

Euploidy (having complete sets of chromosomes) can be advantageous because it often increases cell and organ size and vigour. In plants, polyploid euploids commonly produce larger flowers and fruits, higher yield and useful traits (e.g., seedless triploids). In animals euploidy effects vary; some polyploids show desirable traits but many polyploid animals are inviable or sterile.

Crosses involving inheritance of only one pair of contrasting characters are called monohybrid cross.
Cross between pure tall (TT) plant and Pure dwarf (tt) plant
Parental Generation : Pure breeding Tall and dwarf plants are crossed (TT x tt).
F1 Generation : Monohybrids are heterozygous tall.
F2 Generation : Selfing of the F1 monohybrids takes place. Tall and dwarf plants are obtained in the ratio of 3:1 which is the phenotypic ratio. Genotypically plants are of three types as shown above and therefore genotypic ratio is 1:2:1.

(i) The chromosomes are thin, long and thread like structures consists of two identical strands called sister chromatids.
(ii) They are held together by the centromere.
(iii) Each chromatid is made up of spirally coiled thin structure called chromonema. The chromonema has number of bead-like structures along its length which are called chromomeres.
(iv) The chromosomes are made up of DNA, RNA, chromosomal proteins (histones and non-histones) and certain metallic ions. These proteins provide structural support to the chromosome.
A chromosome consists of the following regions:
Primary constriction : The two arms of a chromosome meet at a point called primary constriction or centromere. The centromere is the region where spindle fibres attach to chromosomes during cell division.
Secondary constriction : Apart from the primary constriction, some chromosomes possess secondary constriction at any point of the chromosome. They are known as the nuclear zone or nucleolar organizer (formation of nucleolus in the nucleus).
Telomere : The end of the chromosome is called telomere. It maintains and provides stability to the chromosomes.
Satellite : Some of the chromosomes have an elongated knob-like appendage at one end of the chromosome known as satellite. The chromosomes with satellites are called as the sat-chromosomes.

Dihybrid cross involves the inheritance of two pairs of contrasting characteristics (or contrasting traits) at the same time. The two pairs of contrasting characteristics chosen by Mendel were shape and colour of seeds: round-yellow seeds and wrinkled-green seeds.
(i) Mendel first crossed pure breeding pea plants having round-yellow seeds with pure breeding pea plants having wrinkled-green seeds and found that only round-yellow seeds were produced in the first generation (F1).
(ii) From this it was concluded that round shape and yellow colour of the seeds were dominant traits over the wrinkled shape and green color of the seeds.
(iii) When the hybrids of F1 generation pea plants having round-yellow seeds were cross-bred by self pollination, then four types of seeds having different combinations of shape and color were obtained in second generation or F2 generation. They were round yellow, round-green, wrinkled yellow and wrinkled-green seeds.
(iv) The ratio of each phenotype (or appearance) of seeds in the F2 generation is 9:3:3:1. This is known as the Dihybrid ratio.
(v) From the above results it can be concluded that the factors for each character or trait remain independent and maintain their identity in the gametes. The factors are independent to each other and pass to the offsprings (through gametes).
Results of a Dihybrid Cross: Mendel got the following results from his dihybrid cross:
(i) Four types of plants: A dihybrid cross produced four types of F2 offsprings in the ratio of 9 with two dominant traits, 3 with one dominant trait and one recessive trait, 3 with another dominant trait and another recessive trait and 1 with two recessive traits.
(ii) New combination: Two new combinations of traits with round green and wrinkled yellow had appeared in the dihybrid cross (F2 generation).
P. Herence between dihybrid of monohybrid uses :
(i) Dihybrid cross is different from a monohybrid cross because it involves inheritance of two pairs of contrasting characters. In monohybrid cross, only inheritance of 1 pair of contrasting character is studied.
(ii) Further results of monohybrid cross is based on two alleles of a gene.
(iii) Results of dihybrid cross is based on two different genes controlling two different characters and their alleles.

DNA is a polynucleotide made of nucleotides (deoxyribose + phosphate + nitrogenous base). It is a double helix of two antiparallel strands with sugar‑phosphate backbones and complementary base pairing (A–T with two hydrogen bonds; C–G with three hydrogen bonds). Each helix turn is 34 Å (3.4 nm) and contains 10 base pairs. DNA stores and transmits hereditary information, contains the code for protein synthesis, and controls development and cellular activities.

(i) Human beings have 23 pairs of chromosomes out of which 22 pairs are auto somes and one pair (23rd pair) is the sex chromosome.
(ii) The female gametes or the eggs formed are similar in their chromosome type (22 + X). Therefore, human females are homogametic.
(iii) The male gametes or sperms produced are of two types.
(iv) The sperm bearing (22+X) chromosomes and the sperm bearing (22+Y) chromosomes. The human males are called heterogametic.
(v) It is a chance of probability as to which type of sperm fuses with the egg.
(vi) If the egg (X) is fused by the X-bearing sperm an XX individual (female) is produced.
(vii) If the egg (X) is fused by the Y-bearing sperm an XY individual (male) is produced.
(viii) The sperm, produced by the father, determines the sex of the child. The mother is not responsible in determining the sex of the child.

a. What do the plants of F1 generation look like?
b. What is the ratio of tall plants to dwarf plants in F2 generation?
c. Which type of plants were missing in F1 generation but reappeared in F2 generation?
(a) The plants of the F1 generation will be tall.
(b) The ratio of tall: dwarf plants (phenotypic ratio) will be 3:1 in the F2 generation.
(c) The- dwarf plants were missing in the F1 generation but reappeared, in F2 generation.

No. The statement given by her family members is false. The birth of male or female babies is by random combination of a sperm and egg. Sperms are of two kinds based on sex chromosome (X chromosome &Y chromosome). The eggs always have only X chromosome (sex chromosomes).
If a sperm with X chromosome fuses with an egg, a girl baby is born.
If a sperm with Y chromosome fuses with an egg, a boy baby is born. The sex of the baby depends on the type of sperm which fertilizes the ovum.