1. Lecture 40 – Lecture 41 – Lecture 42 Mendelian Genetics Ozgur Unal
NIS - BIOLOGY
Lecture 40 – Lecture 41 – Lecture 42
Mendelian Genetics
Ozgur Unal

2. Gregor Mendel What do you think this person’s occupation was?
Gregor Mendel ( ) was an Austrian
monk and plant breeder.
He studied the traits of garden pea plants.
He published his findings on the method
and the mathematics of inheritance.
The study of genetics (science of heredity)
began with Mendel.

3. Pea Plants Pea plants are easy to grow.
Pea plants usually reproduce by self-fertilization.
Self-fertilization occurs when a male gamete within a flower combines with a female gamete in the same flower (sexual or asexual?).
Mendel discovered that pea plants
can easily be cross pollinated by hand.
He transferred a male gamete from the
flower of one pea plant to the female
reproductive organ in a flower of another
pea plant.
He analyzed the results of his experiments and formed hypothesis concerning how the traits were inherited.

4. The Inheritance of Traits

5. The Inheritance of Traits
Some pea plants always produce yellow seeds.
Some pea plants always produce green seeds.
Mendel cross pollinated these two plants and got a new generation of pea plants.
Mendel called the green-seed
plant and the yellow-seed
plant the parent
generation  P generation.

6. The Inheritance of Traits
The new generation of plants from the parent generation has all yellow seeds.
This new generation is called first filial (F1) generation.
The green-seed trait seem to
have disappeared in F1
generation.
Mendel planted the F1
generation of yellow seeds, allow
the plants grow and self fertilize
 Second filial (F2) generation
Of the seeds he collected, 6022
were yellow and 2001 were green
 3 : 1 ratio.

7. Genes in Pairs
Mendel concluded that there must be two forms of the seed trait in the pea plants: Yellow-seed and green-seed.
Each form is controlled by a factor called an allele.
An allele is defined as an alternative form of a single gene passed from generation to generation.
The gene for yellow seeds and the gene for green seeds are each different forms of a single gene.
Mendel called the form of the trait that appeared in the F1 generation dominant (yellow seed) and the form of the trait that was masked in the F1 generation recessive (green seed).

8. Dominance
The allele for the yellow-seed form of the trait is represented by Y (dominant).
The allele for the green-seed form of the trait is represented by y (recessive).
An organism with two of the same alleles for a particular trait is homozygous for that trait.
Homozygous yellow-seed plants  YY
Homozygous green-seed plants  yy
An organism with two different alleles for a particular trait is heterozygous for that trait.
Heterozygous yellow seed plants  Yy
When aleles are present in the heterozygous state, the dominant trait will be observes.

9. Dominance

10. Dominance

11. Genotype and Phenotype
A yellow-seed plant can be heterozygous or homozygous for the trait form  YY or Yy
The outward appearence of an organism does not always indicate which pair of alleles is present.
The organism’s allele pairs are called its genotype.
The observable characteristic or
outward expression of an allele pair
is called the phenotype.

12. Mendel’s Law of Segregation
Check out Figure 10.9
Mendel’s law of segregation states that the two alleles for each trait separate during meiosis.
During fertilization, two alleles for that trait unite.
All resulting F1 generation plants
will have the genotype Yy and will
have yellow seeds because yellow
is dominant to green.
These heterozygous organisms
are called hybrids.

13. Monohybrid Cross A cross that involved hybrids
for a single trait is called a
monohybrid cross.
The Yy plants produce two
types of gametes.
The combining of these
gametes is a random event.
In Mendel’s F1 cross, there
are three possible genotypes:
YY, Yy and yy  genotypic ratio
is 1 : 2 : 1.
The phenotypic ratio is 3 : 1.

14. Dihybrid Cross
Once Mendel established inheritance pattern of a single trait, he began to examine simultaneous inheritance of two ro more traits in the same plant.
Round seeds (R) are dominant ro wrinkled seeds (r)
Yellow seeds (Y) are dominant to green seeds (y)
Homozygous yellow-round seed pea plants  YYRR
Homozygous green-wrinkled seed pea plants  yyrr
P cross  YYRR x yyrr
F1 generation genotype from this P cross  YyRr
F1 generation phenotype from this P cross  Yellow round
The F1 generation plants are called dihybrids because they are heterozygous for both traits.

15. Dihybrid Cross

16. Law of Independent Assortment
Mendel allowed F1 pea plants with the genotype YyRr to self fertilize in a dihybrid cross.
He calculated the genotypic and phenotypic ratios of the offspring in both F1 and F2 generations.
Law of independent assortment states that a random distribution of alleles occurs
during gamete formation.
Genes of separate
chromosomes sort independently
during meiosis.
Check out Figrue 10.11

17. Punnett Squares
In the early 1900s, Reginald Punnett developed a Punnett square.
A punnett square predicts the
possible offspring of a cross
between two known genotypes.
Punnett Square – Monohybrid
Cross:
Yy  Y and y gametes
Punnett square shows the
possible combinations of each
male and female gamete.

18. Punnett Squares Punnett Square – Dihybrid Cross:
YyRr  YR, Yr, yR or yr
The Punnett square
shows the possible
combinations of these
fours gametes from
male and female.

19. Punnett Squares http://www.zerobio.com/drag_gr11/mono.htm
Example: Can you roll your tongue?
Ability to roll tongue (T) is a dominant trait over the inability to roll tongue (t).
Draw the Punnett Square
for the possible
combinations of the
children for a given
genotype of the parents.