Introduction to Genetics

Maternal

B b

Paternal

B BB Bb

b Bb bb

Patterns of Inheritance

Father and son

sisters

brothers

twins

FamilyMom and offspring

Diploid – 2complete sets of chromosomes and 2 complete sets of genes

Haploid – 1 setof chromosomesand 1 set of genes

Homologous chromosomes: Same genes, but different versions, in the same location, each coming from one parent

Crossing Over: Results in the exchange of alleles between chromosomes resulting in a new combination of alleles

Some genes appear to be inherited together, or “linked.” If two genes are found on the same chromosome, does it mean they are linked forever? Study the diagram, which shows four genes labeled A–E and a–e, and then answer the questions on the next slide.

1. In how many places can crossing over result in genes A and b being on the same chromosome?

2. In how many places can crossing over result in genes A and c being on the same chromosome? Genes A and e?

3. How does the distance between two genes on a chromosome affect the chances that crossing over will recombine those genes?

1.One (between A and B)

2. Two (between A and B and A and C); Four (between A and B, A and C, A and D, and A and E)

3. The farther apart the genes are, the more likely they are to be recombined through crossing over.

Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original.

Prophase II Metaphase II

Anaphase II Telophase II

The chromosomes line up in a similar way to the metaphase stage of mitosis.

The sister chromatids separate and move toward opposite ends of the cell.

Meiosis II results in four haploid (N) daughter cells.

* Austrian monk* Teacher of high school natural

science love of evolution, nature, meteorology

* “for the fun of it”: crossed peas and mice- saw inheritance patterns

* pea plants- a formal test

1. The research pea plants- why?

- structure (male and female parts on same plant)

- distinctive traits- rapid reproduction- ability to control pollination and

fertilzation

Used “true-breeding”

plants

-if self-pollination

occurs, offspring

produced that are

identical to parent

**Cross-pollination

allowed for him to

prevent self-pollination

and study the results

1.   Mendel studied the inheritance of one trait (for example plant's height, color of  flowers or color and shape of seeds).

2.    Mendel first cross pollinated tall pea plants (identified asTT, height of plants in this variety were about six feet tall) with each other. 3.   Mendel then cross pollinated short pea plants (identified as tt, height of plants in this variety were about one foot tall) with each other.

X

In every generation of this plant only short plants were produced. He concluded that the pea plant must contain some factor for height (in that variety - for shortness).

4.   The next step of Mendel's experiment was to crossed tall pea plants (TT) with short pea plants (tt). The resulting plants were labeled Tt and only tall plants were produced.

F1 Generation – had the recessive alleles disappeared?

F1 generation crossed by self pollination to create F2 generation

Tt TtX

Tt

Tt

TT TtTt tt F2

Segregation

F1

Alleles segregate from each other so that each gamete carries only a single copy of each gene. Alleles pair up again when gametes fuse during fertilization

Mendel’s next question: Does the segregation of one pair of alleles affect the segregation of another pair of alleles?EX: Does the gene that determines whether a seed is round or wrinkled in shape have anything to do with the gene for seed color?

Conclusion: genes that segregatedon’t influence each others inheritance. This is why we have so many genetic differences in plants and animals.

1. Incomplete Dominance – some alleles are neither dominant or recessive

2. Codominance: both alleles contribute to the phenotype

3. Multiple Alleles: Genes that have more than two alleles.

EX: Blood type exists as four possible phenotypes: A, B, AB, & O. There are 3 alleles for the gene that determines blood type. (Remember: You have just 2 of the 3 in your genotype --- 1 from mom & 1 from dad). The alleles are as follows:

  ALLELE

IA IB i

CODES FOR Type "A" Blood

Type "B" Blood

Type "O" Blood

GENOTYPES IAIA IAi

RESULTING PHENOTYPES

Type A Type A

IBIB IBi

Type B Type B

IAIB Type AB

ii Type O

1.A woman with Type O blood and a man who is Type AB have are expecting a child.  What are the possible blood types of the kid? 

Step #1, figure out the genotypes of ma & pa using the given info. "Woman with Type O" must be ii, because that is the one & only genotype for Type O.

"Man who is AB" must be IAIB, again because it is the one & only genotype for AB blood. So our cross is: ii x IAIB.  The proper p-square would look like this:  

                                                   

As you can see, our results are as follows: 50% of kids will be heterozygous with blood Type A 50% will be heterozygous with blood Type B

What allele combinations can a chinchilla rabbit have?

What are the possible offspring that would result when a chinchilla rabbit mates with a himalayan rabbit?

Single gene, four different alleles

C = full color dominant

Cch=chinchilla; dominant to ch & c

Ch=Himalayan; dominant to C c=albino; recessive

Exceptions to Mendel’s Principles

Sex-linked Inheritance: The X and Y chromosomes in mammals do not have exactly the same genes.

In mammals, the Y-chromosome is small and does not carry many genes.

Sex-linked genes linkedto X chromosome

X-linked Inheritance

Color blindness is one example of X-linked inheritance.

Polygenic Inheritance: A characteristic controlled by more than one gene (many genes shaping one phenotype)

Human Examples:Skin ColorEye ColorHeight

Exceptions to Mendel’s Principles

Polygenic Inheritance: Human Skin Color

Dominant allelesO aabbcc = very light skin

1-2 AABbcc = mediumAaBbCcAaBBcc, etc.

3 AABBCC = very dark skin

AaBbCc x AaBbCc