Mendel mono hybrid

1. Mendel’s Legacy
Genetics is everywhere these days – and it will continue as a dominant force in
biology and society for decades to come.
Wouldn’t it be nice if people understood it better?

2. The Fundamental Question
What is the relationship between genes (genotype) and observable
characteristics (phenotype)?
The answer?
Phenotype = Genotype + Environment.

3. Genes and Environment Determine Characters
Genetically identical hydrangeas growing in soils of different acidity
(different environments).
The phenotype = genotype + environment principle applies equally to
human traits.

4. Dangerously Ahead of the Game - A Eugenics Exhibit at the 1920
Kansas State Fair
Can history be
repeated?

5. A Mendelian Genetic Primer
Genes come in pairs that separate in the formation of gametes.
The members of the pair may be identical (homozygous) or non-identical
(heterozygous).
Each form of a particular gene is an allele.

6. A Mendelian Genetic Primer
One allele is dominant over another (or so Mendel believed).
Only two alleles of a given gene are possible in an individual although many
alleles of a gene are possible within a population.

7. Genes, Alleles, and Chromosomes

8. Mendel’s Model
Organism – the
Garden Pea

9. The Reality of “Round and Wrinkled” – Two Alternative Traits of
the Seed Shape Character
Note that each of
seed is a new
individual of a
different
generation –
seeds are not of
the same
generation as the
plant that bears
them.

10. Mendel’s Monohybrid
Cross – P to F1
A Punnett square,
something we’ll
cover in a moment.

11. Staying the Course – Mendel
Continued Crosses to the F2
(the grandchildren)
What was learned?
The green trait was not lost or
altered, even though it disappeared
in the F1.
One trait is dominant to the other
in its expression.
The reappearance of the recessive trait in ¼ of the F2, suggests genes come in
pairs that separate in the formation of sex cells.

12. Monohybrid Crosses and the Principle of Segregation
A cross between individuals differing in single
character is a monohybrid cross.
The analysis of monohybrid crosses allowed
Mendel to deduce the Principle of Segregation ....
Genes come in pairs that separate in the
formation of sex cells (and these sex cells unite
randomly at fertilization).

13. Principle of Segregation Demystified
The principle of segregation is explained by the behavior of homologous
chromosomes at meiosis.
Segregation

14. A Punnett Square is a Handy Way of Analyzing Crosses
In a Punnett square for a monohybrid cross, the Principle of Segregation is applied.

15. Different Genotypes Can Produce the Same Phenotype

16. Consistency is
Good
Characters
investigated by
Mendel
No matter what the
character, Mendel observed
a 3:1 ratio of characters in
the F2.

17. Monohybrid Crosses Yielded Consistent Results
Therefore, the Principle of Segregation indeed is a general principle of genetics.

18. What Works for Peas Also Works for Humans
An albino woman
In the cross Aa x Aa, where A is a
dominant allele for wild type (standard)
pigmentation and a is a recessive allele
for no pigmentation (albinism), ¾ of
offspring will be wild type and ¼ will be
albino.

19. Do this monohybrid cross
• In pea plants, round seeds are dominant to
wrinkled seeds. If a heterozygous round
seed plant is crossed with a heterozygous
round seed plant, what is the expected
phenotypic and genotypic ratios in the F1?

20. Answer
R r
R
r
RR Rr
Rr rr
Phenotypic Ratio: 3 round : 1 wrinkled
Genotypic ratio: 1 RR : 2 Rr : 1 rr
P – Rr x Rr
Alleles produced for gametes are R and r from each parent

21. Try This one !
Fruit fly wing length is controlled by a
dominant allele for long wings (L). If a
heterozygous long winged fly is mated to a
homozygous long winged fly, what is the
expected phenotypic and genotypic ratio in
their offspring?