IntroductionIntroductionHeredity: tendency of an individual

to resemble his parents

Variation: the differences between individuals of the same species

Genetics: the study of how characters are transferred or inherited from one generation to the next

Mendel’s Breeding ExperimMendel’s Breeding ExperimententMonohybrid Inheritance:

The inheritance of just one pair of contrasting charactersCross 2 pure breeding parents with contrasting characters

whitepurple

xpure-breeding parents

first filial (F1) generation

second filial (F2) generation

all purple

705 purple

224 whitemonohybrid ratio 3:1

TermsTerms Genotype - the genetic composition Phenotype - the external appearance

Homozygous (pure-breeding)– with the same alleles in the same

character Heterozygous (hybrid)

– with different alleles

TermsTerms• Dominant gene (or character)

– which expresses its effect even in heterozygous condition

• Recessive gene (or character)– which expresses its effect only in homozygous condition

• Pedigree– it shows the inheritance of one or more

characters in different generations

TermsTerms• Test cross

– cross the organism with dominant character to an organism with homozygous recessive character

– to test whether the organism with the dominant character is homozygous or heterozygous

• Back cross– cross an organism with one of its

parent

Sex DeterminationSex Determination• in human :

by sex chromosomes

• male : XY• female : XX

• chromosomes other than sex chromosomes somatic chromosomes which control normal body characters

XY XX

• 22 pairs are identical in both sexes (human)– autosomes

• The 23rd pair is different in male and female– sex chromosomes

TwinsTwins• identical twins

– one embryo separates into two and each develops into a new individual

– they should have same characters and same sex

• non-identical twins– formed from two different zygotes from two

separate fertilizations – they usually have different characters– they may have different sexes

human height

continuousvariation

• continuous variation– with a continuous range of intermediate valu

es– eg. height, weight, intelligence quotient, hear

t beat

No.

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Types of VariationsTypes of Variations

Types of VariationsTypes of Variations• discontinuous variation

– the character is clear-cut & not showing intermediates

– eg. tongue rolling, ear lobes and colour of corn

albinism

• Meiosis

– homologous chromosomes separate independently from each other and pass into a different gamete ( INDEPENDENT ASSORTMENT). As a result, a great variety of gametes are produced

• Random Fertilization– since fertilization is a random process,

there are many possible different combinations of genes in a zygote

Causes of VariationsCauses of Variations

Causes of VariationsCauses of Variations• Mutation

– genetic make-up may suddenly change– sometimes occur naturally– rate may greatly increased if the

organism is exposed to radiation, certain chemicals or neutron bombardment

– most mutation are harmful

• Environmental Factors– cause variation in characters with

continuous variation

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