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Rules for the Inheritance of Traits - Mendel’s Contributions

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How are traits expressed

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RULES FOR THE INHERITANCE OF TRAITS- MENDEL’S CONTRIBUTION:-

-Gregor Johann Mendel started his experiments on plant breeding and hybridisation. He proposed the laws of inheritance in living organisms.

-Both the father and the mother contribute practically equal amounts of genetic material to the child. This means that each trait can be influenced by both paternal and maternal DNA.

- for each trait there will be two versions in each child.

-Pisum Sativum (garden pea) was the plant selected by Mendel for his experiments.

-he use a number of contrasting characters for garden pea.

Monohybrid Cross:-

Mendel used a number of contrasting visible characters of garden peas – round/wrinkled seeds, tall/short plants, white/violet flowers and so on. He took pea plants with different characteristics – a tall plant and a short plant, produced progeny by crossing them, and calculated the percentages of tall or short progeny.

In the first place, there were no halfway characteristics in this first generation, or F1 progeny – no ‘medium-height’ plants. All plants were tall. This meant that only one of the parental traits was seen, not some mixture of the two. Mendelian experiments test this by getting both the parental plants and these F1 tall plants to reproduce by self-pollination. The progeny of the parental plants are, of course, all tall. However, the second-generation, or F2, progeny of the F1 tall plants are not all tall. Instead, one quarter of them are short. This indicates that both the tallness and shortness traits were inherited in the F1 plants, but only the tallness trait was expressed. This led Mendel to propose that two copies of factor (now called genes) controlling traits are present in sexually reproducing organism. These two may be identical, or may be different, depending on the parentage.

Pure or homozygous condition:-

(TT, tt): Both are dominant, Both are recessive.

Heterozygous condition(Hybrid):-

Tt: One is dominant, one is recessive

Phenotype means physical appearance either they are Tall or Short.

Genotype means genetic make up that are TT, Tt or tt.

Phenotypic ratio: - 3:1 (three tall and one short)

Genotypic ratio: - 1:2:1 (TT-one, Tt-two, tt-one)

Conclusions:-

-’TT’ & ‘Tt’ both are tall plants while ‘tt’ is short.

-A single copy of ‘T’ is enough to make the plant tall while both copies of ‘t’ have to be present for the plant to be short..

-Traits like ‘T’ are called dominant traits because it expresses itself and ‘t’ are recessive traits because it remains suppressed.

Dihybrid cross:-

It is the cross between two plants which have two pairs of contrasting characters. This takes into consideration alternative traits of two different characters. For example, a cross between one pea plant with round and green seeds and the other pea plant having wrinkled and yellow seeds.

 

RY

Ry

rY

ry

RY

RRYY

RRYy

RrYY

RrYy

Ry

RRYy

RRyy

RrYy

Rryy

rY

RrYy

RrYy

rrYY

rrYy

ry

Rryy

Rryy

rrYy

rryy

Phenotypic ratio:-

Round, Yellow: 9

Round, Green: 3

Wrinkled, yellow: 3

Wrinkled, green: 1

 

Observations:-

(i)when RRyy was crossed with rrYY in F1 generation all were Rr Yy round and yellow seeds.

(ii)Self pollination of F1 plants gave parental phenotype and two mixture seeds plant in the ratio of 9:3:3:1

(iii) Round and yellow seeds are dominant characters.

(iv) occurrence of new phenotype combinations show that genes for round and yellow seeds are inherited independently of each other.

HOW THESE TRAITS GET EXPRESSED:-

-Cellular DNA is the information source for making proteins in the cell.

-A section of DNA that provides information for one protein is called the gene for that protein.

-Plant height can thus depend on the amount of a particular plant hormone. The amount of the plant hormone made will depend on the efficiency of the process for making it.

-If this enzyme works efficiently, a lot of hormone will be made, and the plant will be tall. If the gene for that enzyme has an alteration that makes the enzyme less efficient, the amount of hormone will be less, and the plant will be short. Thus, genes control characteristics, or traits.

Shaalaa.com | Heredity and evolution part 3 (Mendel's Experiment)

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