Module II
Mendelian Genetics &
Probability Theory
SEXUAL REPRODUCTION
involves
• Manufacture of gametes (gametogenesis) through the
process of meiosis (consists of 2 specialized cell divisions
in which the chromosome number of the resulting cells is
reduced from diploid (2n) to a haploid (n) number)
• Gametogenesis occurs in specialized cells in the
reproductive organs (gonads)
• In animals, testes are male gonads and ovaries are female
gonads
• The union of male and female gametes (fertilization)
produces a zygote
• In humans, male gametes are sperms and female gametes
are ova
GENETICS
• Genetics is the study of heredity and variation in
organisms
• Genes are the hereditary units transmitted from
one generation to the next
• Genes reside in the long molecules of
deoxyribonucleic acid (DNA) that exists within
the cell
• DNA, in conjunction with a protein matrix forms
nucleoprotein and becomes organized into
structures called chromosomes that are found in
the nucleus of the cell
ALLELES
• Mutation is a change in some part of the DNA
• The result of a mutation is a change of character or
trait of the individual
• Through a mutation, a gene may be changed into
two or more alternative forms called alleles
• Homologous chromosomes are genetically similar
MENDEL’S LAWS
• Law of segregation: If a parent contains 2
alleles of a trait, only 1 is transmitted
through a gamete to the offspring
• Law of independent assortment:
Segregation of one gene pair occurs
independently of any other gene pair
MENDELIAN INHERITANCE
(Assumes Mendel’s Laws and independence of environmental conditions)
•
Phenotype – traits possessed by an organism (
may be visible, like color)
• Genotype – All alleles of a gene possessed by
an individual. Classified as:
A) Homozygous – union of gametes carrying
identical alleles; produce homozygotes
B) Heterozygous – union of gametes carrying
different alleles; produce heterozygotes
DOMINANT & RECESSIVE ALLELES
•
A Dominant allele of a gene dictates the phenotype of the
organism. Indicated by a capital letter, a homozygous
dominant individual could have a genotype shown as AA.
A heterozygous genotype would be shown as Aa to
indicate the presence of a recessive allele form of the gene.
• A Recessive allele does not determine phenotype in the
presence of a dominant allele. Indicated by a lower case
letter. A homozygous recessive individual could have a
genotype shown as aa.
• A heterozygous individual who possesses a recessive allele
is called a carrier
SIX MONOFACTORIAL CROSSES
(summarized in table below)
Expected ratios of first filial generation
Cross
No.
1
2
3
BB X bb
Bb X Bb
BB X BB
4
5
6
bb X bb
Bb X BB
Bb X bb
Mating
Genotypes
Phenotypes
All Bb
All black
1 BB : 2 Bb : 1 bb
3 black : 1 white
All BB
All black
All bb
All white
1 BB : 1 Bb
All black
1 Bb : 1 bb
1 black : 1 white
