Transcript Dominant

Chapter 9
Patterns of Inheritance Mendel and His Discoveries
Gregor Mendel
Father of Genetics
In the years 1856 – 1863 carried
out his experiment
– Genes are arranged in the
chromosomes
– Genes are responsible for
passing the parental
characters (traits) to the
offsprings
– 2 genes are responsible for
each characteristic.
– These 2 genes separate at
the time of gamete formation
– Heredity - the transmission of genetic traits
from one generation to the next.
– Locus
- the location of a gene on
chromosome.
– Alleles -Two pairs of genes which will be
responsible for particular trait .
– Genetics - the study of heredity - explains
how characteristics are passed
on from parent to child.
• Mendel conducted his experiments using garden
pea plants (Pisum sativum)
• He applied mathematics to his observations, and
could soon predict the outcome of his
experiments
Reason for selecting pea plant
• Can be self or cross pollinated ( the deposition
of pollen grain on the stigma is pollination)
• Can be grown quickly (easily cultivated)
• They are annuals - complete their life cycle
within one year; all the stages of growth
observed within a year
• Note:
Self pollination – pollen grain of a flower is
deposited on its own stigma.
Cross pollination – pollen grain of one flower
is deposited on the stigma of
another flower.
Self Pollination
Pollen from the
anther (male) of a
plant falls on the
stigma (female) of
that same plant.
Cross Pollination (Manual)
Pollen from the anther
(male) of one plant is
transferred to the stigma
(female) of another plant
of the same species.
Phenotype
• External appearance or the outward
appearance of a living organism.
• Example:
–Tall plant
–Dwarf plant
–Wrinkled seed
–Smooth seed
–Colored seed
Genotype
• Genetic makeup of a living organisms.
• Examples:
– TT
– tt
– SS
– Ss
– YY
– yy
Dominant and
Recessive Traits
In his studies, Mendel
recognized physical
characters of pea
plants. There were
seven characteristics
for seed and flower
color, seed shape and
plant height.
Parental, F1 and F2 Generations
• Parental Generation
– P
• Offspring of parental generation
– F1 (first filial generation)
• Offspring of first filial generation
– F2 (second filial generation)
• When an organism has two identical
alleles (genes) for a given trait, the
organism is Homozygous.
– For example TT and tt
• If an organism has two different alleles
(genes) for a given trait, the organism is
Heterozygous.
– For example Tt
Monohybrid Cross
• Mendel took single trait for crossing i e.,
height of the plant.
• He crossed two true-breeding (pure),
homozygous plants (parents) having
contrasting traits
– crossed, homozygous (pure) tall plant (TT)
with homozygous (pure) dwarf plant (tt), all
the individuals of F1 generation were tall and
heterozygous (Tt) in condition.
• In heterozygous condition ( T t ), one trait
(T) masked the effect of the other (t)
- the expressed character is Dominant and the
one not expressed is Recessive. Dominant
allele “T”, masked the presence of the
recessive allele “t”.
- All the plants in F1 were tall.
F1 individuals of heterozygous tall condition
were intercrossed (Tt x Tt)
- producing F2 generation
1 (TT) : 2 (Tt) : 1 (tt) - F2 genotypic ratio
3Tall : 1 dwarf - F2 phenotypic ratio
Mendel’s P generation & F1
Cross
Homozygous male and female parents with
pure yellow seed and pure green seeds when
crossed resulted in heterozygous yellow
showing only the dominant character in the F1
generation.
Above mentioned cross can be graphically
represented in the Punnett square.
Mendel’s P generation & F1
Cross
From F1 to F2 Generation
When F1 heterozygous yellow seeded plants
were crossed among themselves, the following
monohybrid genotypic ratio is expressed for F2
generation: I homozygous yellow seed (YY) :
2 heterozygous yellow seed (Yy) :
1 homozygous green seed (yy).
Monohybrid phenotypic ratio for F2 generation is
3 yellow seed :1 green seed.
From F1 to F2 Generation
The Law of Dominance
Any one factor may mask the effect of another
factor in a given pair of alleles.
The expressed allele is referred to as dominant,
and the one that is not expressed is recessive.
Example - in the cross between yellow seed and
green seed variety, yellow seed variety is
dominant over the green seed
The Law of Segregation
• Each character is governed by two alleles
– Alleles separate at the time of gamete
formation.
Dihybrid Cross
Dihybrid cross is one in which two characteristics
from each parent were considered for crossing.
In this cross, a parent with pure yellow color
smooth seed is crossed to a parent with pure green
color wrinkled seed. All F1 generation will have
yellow and smooth seeds.
Dihybrid Cross (cont’d)
• When F1 generation plants are intercrossed, F2
generation produced 9 yellow smooth seed : 3
yellow wrinkled seed : 3 green smooth seed :
1 green wrinkled seed.
• Dihybrid phenotypic ratio : 9:3:3:1
• Dihybrid genotypic ratio : 1SSYY : 2SSYy : 1SSyy : 2sSYY : 4SsYY :
2Ssyy : 1ssYY : 2ssYy : 1ssyy
Dihybrid Cross (cont’d)
Law of Independent Assortment
• At the time of gamete formation, alleles
assort independently.
– Dihybrid cross parents with yellow round and
green wrinkled seeds
• produce yellow wrinkled and green round
seeds in F2 generation
Incomplete Dominance
Cases in which characteristics blend
- Snap Dragon Flowers
Red (RR) X White (rr) = 1 Red (RR) : 2 pink
(Rr) : 1 white (rr)
Incomplete Dominance
Co-Dominance
Instances where
neither allele is capable
Of dominating over the
other, but share
(express) equal
contributions to the trait
Multiple Alleles/Polygenic Inheritance
• Multiple Alleles - Three or more Alleles
determine any individual characteristic
– height, weight, eye color, skin color
• Variations within a characteristic often
present in a normal bell curve.
Multiple Alleles/Polygenic Inheritance
Pleiotropy
One gene capable of expressing many
effects
Results
- mental retardation
- various facial anomalies