What is Down's syndrome? Give its symptoms and cause. Why is it that the chances of having a child with Down's syndrome increases if the age of the mother exceeds forty years?
Down's syndrome is a human genetic disorder caused due to trisomy of chromosome number 21 . Such individuals are aneuploid and have 41 chromosomes, i.e., $(2 n+1)$
Symptoms of down's syndrome are
(i) Mental retardation
(ii) Growth abnormalities
(iii) Constantly open mouth
(iv) Dwarfness, etc., gonads and genitalia under developed
The reason for the disorder is the non-disjunction (failure to separate) of homologous chromosome (a pair 21 during meiotic division. The chances of having a child with Down's syndrome increases with the age of the mother ( +40 ) because age adversely affects meiotic chromosome behaviour.
Meiosis in the egg cells is not completed, until after fertilisation. During this long gap (till meiosis is not completed) egg cells are arrested in prophase I and chromosomes are unpaired. The greater the time they remain upaired greater the chance for unpairing and chromosome non-disjunction.
How was it concluded that genes are located on chromosomes?
Chromosome theory of inheritance was proposed by Sutton and Boveri independently in 1902. The theory believes that chromosomes are vehicles of heriditary information, possess Mendelian factors or genes and it is the chromosomes which segregate and assort independently during transmission from one generation to the next.
A plant with red flowers was crossed with another plant with yellow flowers. If F1 showed all flowers orange in colour, explain the inheritance.
Incomplete dominance is the phenomenon where neither of the two alleles shows dominance thus producing intermediate hybrid between the expression of two alleles in homozygous state. In this case, a new phenotype in between the two original phenotype appears.
What are the characteristic features of a true-breeding line?
True breeding is a stable trait inheritance and expression for several generations as a result of continuous self-pollination.
Characteristic features of a true-breeding line
(i) They are used as parents in artificial hybridisation as they provide gametes with all similar traits.
(ii) Homozygous recessive plants are used in test cross to determine the genotype.
In peas,tallness is dominant over dwarfness, and red colour of flowers is dominant over the white colour. When a tall plant bearing red flowers was pollinated with a dwarf plant bearing white flowers, the different phenotypic groups were obtained in the progeny in numbers mentioned against them
$$ \begin{aligned} & \text { Tall, Red }=138 \\ & \text { Tall, } \text { White }=132 \\ & \text { Dwarf, Red }=136 \\ & \text { Dwarf, } \text { White }=128 \end{aligned} $$
Mention the genotypes of the two parents and of the four offspring types.
The result shows that the four types of offspring are in a ratio of $1: 1: 1: 1$. Such a result is observed in a test cross progeny of a dihybrid cross.
The cross can be represented as
Parents Tall and red $(\mathrm{TtRr}) \times$ Dwarf and white (ttrr)
Offsprings
