Download Genetics

Introduction
Heredity: 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 Experiment
Monohybrid Inheritance:
The inheritance of just one pair of contrasting
characters
Cross 2 pure breeding parents with contrasting
characters
pure-breeding
parents
x
purple
first filial (F1)
generation
second filial (F2)
generation
white
all purple
705 purple
224 white
monohybrid ratio 3:1
Terms
 Genotype - the genetic composition
 Phenotype - the external appearance
 Homozygous (pure-breeding)
– with the same alleles in the same
character
 Heterozygous (hybrid)
– with different alleles
Terms
• 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
Terms
• 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 Determination
• in human :
by sex chromosomes
• male : XY
• female : XX
XY
XX
• chromosomes other than sex chromosomes 
somatic chromosomes which control normal body
characters
• 22 pairs are identical in both sexes
(human)
– autosomes
• The 23rd pair is different in male and female
– sex chromosomes
Twins
• 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
Types of Variations
No. of individuals
• continuous variation
– with a continuous range of intermediate values
– eg. height, weight, intelligence quotient, heart beat
continuous
variation
human height
Types of Variations
• discontinuous variation
– the character is clear-cut & not showing
intermediates
– eg. tongue rolling, ear lobes and colour of corn
albinism
Causes of Variations
• 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 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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