Download Mendel`s Example

Science 103 Spring 2006 – Outline 15
Reading:
Mendel and the Principles of Genetics
P 560-561, 563-566
Gregor Mendel (1822-1884)
1. Why was Mendel’s Work so Important?
(a) Principles of Heredity
(b) Quantitative Approach
2. Mendel’s Study Subject
Garden pea – good choice because:
(a) Can self or cross-fertilize.
(b) Produces large numbers of offspring.
(c) Easy to grow.
(d) Has a number of lines with different traits (eg. flower color, seed shape).
(e) Short generation times.
3. Mendel’s Study Design
Crosses were made between pea plants in 3 steps:
(a) Ensured a True-Breeding Parental (P) Generation
Self-fertilized plants for several generations to make sure they were truebreeding.
Eg. White flowered plants  White flowered progeny
(b) Crossed 2 True-Breeding Pea Varieties with Alternate Traits  First Filial (F1)
Generation
Eg. P Generation
White X Purple

F1 Generation
(c) Self-Fertilized the F1 Generation  F2 Generation
1
Principles of Genetics
(These are the basic discoveries of Mendel, written up in terms of current biological
knowledge).
1. Genes
2. Homologous Chromosomes
(a) Diploid individuals receive 2 alleles (forms of a gene) for each trait, one on each of 2
homologous chromosomes.
(b) Terminology
(i) Homozygous
(ii) Heterozygous
3. Alleles
(a) Definition
(b) Dominant and Recessive Alleles
(i)
(ii) (The dominant allele is represented by a capital letter and the recessive allele by a
lowercase letter).
Eg. Dominant –
Recessive –
Heterozygous –
4. Genotype and phenotype
(a) Genotype
(b) Phenotype
2
Mendel’s Experiments
Monohybrid Cross
Read about Mendel’s reasoning – remember that he did not know the mechanism
of heredity.
Used Plants true-breeding for flower color in the P generation:
P Generation (Cross)
Phenotypes used
Purple X White
Genotypes
Gametes
Punnett Square:
Gametes
p
F1 Generation
Genotype
Phenotype
F1 Generation (Cross)
Phenotypes
Genotype
Gametes
Punnett Square:
Gametes
3
F2 Generation
Genotypes
Ratio
Phenotypes
Ratio
Mendel’s Actual Results for the F2 Generation:
705 purple
:
224 white
75.9%
24.1%
Mendel saw this ratio in all 7 traits he examined.
Do the monohybrid cross you will be given in class.
Testcross
1. Description
2. Mendel’s Example
(a) Question (try writing a prediction for the experiment below)
Are the purple flowered plants in the F2 generation homozygous dominant or
heterozygous?
(b) Experiment
Cross the purple flowered plants in the F2 generation with
____________________________________.
(c) Results
Phenotype
Generation
Purple X White
F2
P
(i) When F2 is HomozygousDominant:
Cross
(ii) When F2 is Heterozygous:
PP X pp
Pp X pp
Gametes
4
Progeny Genotype(s)
Ratio
Therefore, an individual that is heterozygous will have progeny in a phenotypic
AND genotypic ratio of _____________.
Dihybrid Cross
The inheritance of pairs of genes.
Mendel’s Example
Examined 2 traits at once, in this example the traits for seed shape and color.
Seed Shape
Phenotype:
Allele:
Seed Color
Round Vs Wrinkled
R
r
Yellow Vs Green
Y
y
P Generation (Cross)
True breeding individuals that were homozygous dominant for both traits with
individuals that were homozygous recessive for both traits.
Phenotype
Genotype
Homozygous dominant X Homozygous recessive
Round, yellow
Wrinkled green
Gametes
Punnett Square:
Gametes
F1 Generation:
Phenotype
5
F1 Generation (Cross)
Phenotypes
Genotypes
Round yellow X Round yellow
Gametes
Punnett Square:
Gametes
Complete the above table on your own.
Figure
Results
Mendel got a 9:3:3:1 ratio:
9/16
3/16
3/16
1/16
Mendel’s Laws
(In “modern” biological Terms)
Mendel’s First Law – Segregation
In Terms of Meiosis:
6
Mendel’s Second Law – Independent Assortment
In Terms of Meiosis:
7