heredity and evolution

traits/ characters

a recognisable feature of a human being or any other organism like height, complexion, shape, color of hair and eyes etc. is called a trait or character

heredity

the transmission of characters or traits from the parents to their offsprings is called heredity

the hereditary information is present in the gametes of the parents. thus, gametes are the link between one generation and the next

variation

the differences in traits among the individuals of a species is called variation

variations produced during asexual reproduction is very small but those produced during sexual reproduction is very large

accumulation of variation and why its necessary

the significance of variation only shows up if it continues to be inherited by the offspring for several generations

the great advantage of variation to a species is that it increases the chances of its survival in a changing environment

chromosomes

chromosome is a thread-like structure in the nucleus of a cell formed of DNA which carries the genes. different organisms have different number of chromosomes in their nuclei

genes

genes are units of DNA/ chromosome which governs the synthesis of proteins, which control a specific character. they are the units of heredity which transfer traits from parents to their offsprings

dominant and recessive genes

dominant genes: the gene that decides the appearance of an organism even in the presence of alternative/ contrasting genes Ex: TT, Tt

recessive genes: the gene that decides the appearance of an organism only in the presence of another identical gene is called recessive gene Ex: tt

genotype and phenotype

genotype is the description of genes present in an organism (TT, Tt, tt)

phenotype is the trait which is visible in an organism (tall, dwarf)

first and second filial generation

when two parents cross to produce progeny (offsprings), then their progeny is called first filial or F1 generation

when the first generation progeny cross among themselves to produce second generation progeny, then this progeny is called second filial or F2 generation

why mendel chose pea plant for his experiments

it is bisexual and self-pollinating

shows rapid multiplication

it has many contrasting features (long stem vs short stem, round-yellow vs wrinkled-green etc.)

law of segregation

two factors of a character (alleles) which are together in an individual separate randomly at the time of gamete formation. as a result, the gamete contains only a single factor of a character and is always pure. this law is also called the law of purity of gametes

monohybrid cross

a monohybrid cross is defined as the cross happening in the F1 generation offspring of parents differing in one trait only

monohybrid ratio in F2 generation; 3:1

dihybrid cross

a dihybrid cross is the cross happening in the F1 generation as parents are differing in two traits

dihybrid ratio in F2 generation; 9:3:3:1

law of independent assortment

in the inheritance of more than one pair of traits in a cross simultaneously, the factors responsible for each pair of traits are distributed independently to the gametes

how blood groups are inherited

the blood groups have the following genotypes;

A: IAIA / IAIO [where IO is recessive gene]

B: IBIB / IBIO [where IO is recessive gene]

AB: IAIB

O: IOIO

sex determination in humans

the process by which the sex of a person is determined is called sex determination. the chromosomes which determine the sex of a person are called sex chromosomes

a male has one X chromosome and one Y chromosome. this means that half the gametes have X chromosome and half have Y chromosome

a female has two X chromosomes. this means that all the gametes carry X chromosome.

the sex of a child depends on what happens during fertilisation. if a sperm carrying X chromosome fertilises the egg, the child is a girl and if a sperm carrying Y chromosome fertilises the egg, the child is a boy

sex determination in other animals

in some reptiles, the temperature at which the egg is incubated determines the sex of the offspring. in turtles, higher incubation temperature leads to the development of female offspring while in lizards, higher incubation temperature leads to the development of male offspring