1. Genetics, Part II: Mendelian Genetics
Topic 10

2. Mendel first demonstrated the principles of inheritance
Other plant breeders at the time of Mendel knew two main facts
Hybrid parents with the same two kinds of parents are similar
When hybrid offspring are self-fertilized, their offspring show a variety of traits
Mendel did his experiments primarily on the garden pea
Mendel developed true breeding lines, whose offspring all produced the same visible traits (phenotype) as their parents
Mendel then chose 7 clearly identifiable pairs of contrasting traits

3. Mendel first crossed true breeding plants with contrasting traits
a. These parental (p generation) plants produced the first fillial (f1) generation b. The f1 generation plants all possessed the trait of only one of the two parents
c. When the f1 plants self-pollinated the resulting f2 generation had characteristics of both p generation plants
d. These experiments showed that the hereditary factors had not been lost in the f1 generation, but were somehow masked

4. Mendel’s Principles
1. The law of segregation states that alleles separate before gametes are formed
a. the alleles from the male and female gametes do not mix in any way
b. segregation is due to the separation of homologous chromosomes during meiosis
2. Principle of independent assortment states that alleles of different loci on nonhomologous chromosomes are randomly distributed to gametes
a. the exception to this is linked alleles
b. the principle of independent assortment is illustrated by dihybrid crosses, which will be explained once a review of monohybrid crosses has been done

5. Dihybrid Crosses
A. Involves individuals that have different alleles of two loci
B. If a dihybrid cross involves different loci on non-homologous chromosomes, independent assortment occurs
1. This is supported by Mendel’s theories!
C. The mechanics of meiosis are the basis for independent assortment
1. This process occurs during metaphase I of meiosis
Lets try a dihybrid cross of 2 Heterozygous Round-Yellow pea plants

7. Dihybrid Crosses
Homozygous Tall- Round plant vs. Heterozygous Tall – Round plant
TtYY - ttyy

8. Linkage Group:
·all the genes located on the same chromosome

9. linked genes are more likely to be passed together (exceptions to independent assortment)
The closer two genes are on the chromosome, the less likely it is that they will be separated during crossing over in prophase I

10. Linked Genes
Linked genes do not assort independently
1. The principle of independent assortment does not apply to loci on the same homologous pair of chromosomes
2. Crossing over may skew the proportions of expected phenotypic frequencies

11. In dihybrid crosses of traits that are linked, offspring that have a different combination of alleles than their parents are called recombinants
1. Recombinant children are produced as a result of crossing over despite the proximity of the linked genes
2. For example, if the dominant alleles for two genes are linked and the recessive alleles for the genes are linked the recombinant offspring would have chromosomes that contained the dominant allele for one gene and the recessive for the other
3. Because the genes are linked, the recombinants will occur in much smaller frequency than offspring that carry chromosomes that are identical to their parents (for the two linked genes)

12. Linked Gene Test Cross: Dihybrid cross between a known heterozygous (F1) and a homozygous recessive to determine gene linkage.
B = Brown Body
b = Black Body
E = Red Eyes
e = Brown Eyes
This cross shows expected results for independent assortment.

13. B = Brown Body
b = Black Body
BbVv x bbvv
V = Long Wings
v = Vestigial wings
·phenotypic ratios do not follow the expected pattern for independent assortment.
·indicates that the genes are part of the same linkage group.
BbVv
Bbvv
bbVv
bbvv

14. Linked genes are depicted on a horizontal line that represents the chromosome.
a b
A B