Warning-These notes are provided only as a guide. They may not be exactly what is used in class. Due to formating for HTML they may also be less clear than might be desired. Use them at your own risk. Dr. G.

Genetics

Gregor Mendel, 1800s

-did 1st scientific study

-knew nothing of chromosomes or cell division

-derived important laws from his work

{Notes on Mendel}

DISCOVERING THE LAW OF DOMINANCE

-Mendel noticed that pea plants came in traits of 2 forms

-some plants "bred true" for certain traits

-these were self- pollinating plants

seed shape

round, wrinkled

seed color

yellow, green

seed coat color

colored, white

pod shape

inflated, constricted

pod color

green, yellow

flower position

axial, terminal

stem length

long, short

-Mendel cross-pollinated these pure breeders

-these were called the parent, or P1 generation

pure tall X pure short -- > all tall

-the "short" trait disappeared in the first filial, or F1 generation

-Mendel got the same kinds of results for all seven traits

-offspring of crosses are called hybrids

-Mendel wanted to know what happened to "short" trait

-he allowed the F1 generation to self- pollinate

-producing an F2 , or second filial generation

-3/4 tall and 1/4 short

-the "short" trait was not lost

-traits expressed in F1 -- > dominant

-traits hidden in F1 --> recessive

LAW OF DOMINANCE

-IN A HYBRID, ONLY THE DOMINANT TRAIT CAN BE SEEN

Ratio

3:4 dominant in a mono hybrid cross.

-Mendel tried to explain why traits "disappeared"

-he made several hypothesis:

1-each trait was controlled by a different "factor"

2-each "factor" was 1 of 2 kinds

e.g. shortness or tallness

3-each plant had a pair of "factors" which could be alike or different

4-in a cross, offspring received 1 "factor" from each parent

5-dominant, seen, recessive, not seen, but also not lost

DISCOVERING THE LAW OF SEGREGATION

LAW OF SEGREGATION

-PAIRED FACTORS SEPARATED DURING GAMETE FORMATION, RECOMBINE AT FERTILIZATION

GENE

-CHROMOSOME THEORY

-Mendel's work was rediscovered in 1900, more info on cells, mitosis and meiosis

-1903, W.S. Sutton suggested that chromosomes carried Mendel's "factors"

-studied formation of grasshopper sperm

-Sutton published the gene-chromosome theory, 1903

-"factors" were officially called "genes"

FUNDAMENTALS OF GENETICS

review:

alleles-different forms of a gene, controlling a certain trait

e.g. Mendel's -- >height:short, tall

homozygous-identical alleles for a trait

e.g. P generation

heterozygous-different alleles for a trait

e.g. F1 generation

dominant alleles are written in capitals (TT)

recessive alleles are written in lowercase (tt)

genotype-genetic make- up

e.g. tall pure (TT)

tall hybrid (Tt)

phenotype-physical manifestation

e.g. both tall

gamete-haploid cells that fuse to form zygotes

e.g. sperm or pollen

zygote-diploid cell resulting from the fusion of 2 gametes

PROBABILITY OF GENETICS

-probability concerns law of chances

e.g. penny toss: expect 1 head, 1 tail

ratio-1:1

short trials, farther from ratio

longer trials, closer to ratio

(Mendel, worked with 100s of plants)

LAW OF PROBABILITY

NO ONE CHANCE IS MORE LIKELY TO OCCUR THAN THE OTHER

THE PUNNET SQUARE

-consider how pure tall and pure short alleles separate during meiosis, and recombine during fertilization

punnet square-diagram to show expected ratios of genotypes and phenotypes

-males arranged across top

-females along side

-each box indicates a zygote

-law of probability combined with punnet square helps explain Mendel's results

NB: Only works in large #s of offspring

Small #s, only indicate chances of occurrences

PUNNET SQUARE FOR HYBRID CROSS

Tt X Tt -->

expected phenotype:

3/4 tall

1/4 short

(3 tall : 1 short)

expected genotype:

1/4 TT (pure tall)

½ Tt (hybrid tall)

1/4 tt (pure short)

(1 pure tall:2hybrid tall:1 pure short)

THE TEST CROSS

-helps determine genotype from phenotype

test cross-an individual of unknown genotype is mated with an individual showing the contrasting recessive trait

test cross 1:

homozygous tall(test) x homozygous short

-->100% tall- phenotype

-->100% Tt -genotype

test cross 2:

heterozygous tall(test) x homozygous short

-->50% short, 50% tall-phenotype

-->50% Tt

50% tt-genotype

-any recessive phenotypes indicate that the test individual was a hybrid

DISCOVERING THE LAW OF INDEPENDENT ASSORTMENT

-Mendel's 1st experiments involved 1 pair of traits

-then followed 2 pairs of contrasting traits

-did a dihybrid cross for seed color:

pure: yellow round x green wrinkled

-F1 phenotypes were all yellow and round

-F1s were then crossed:

hybrid: yellow round x yellow round

-all phenotypes were seen:

yellow round

yellow wrinkled

green round

green wrinkled

LAW OF INDEPENDENT ASSORTMENT-

GENES FOR DIFFERENT TRAITS ARE SEPARATED AND DISTRIBUTED TO GAMETES INDEPENDENTLY OF ONE ANOTHER

PUNNET SQUARE FOR PURE DIHYBRID CROSS

YYRR X yyrr

PHENOTYPE-100% yellow, round

GENOTYPE-100% hybrid for both traits

PUNNET SQUARE FOR F1 DIHYBRID CROSS

YyRr X YyRr

PHENOTYPE

-9 yellow, round

-3 yellow, wrinkled

-3 green, round

-1 green, wrinkled

NB: PHENOTYPE RATIO 9:3:3:1

INCOMPLETE DOMINANCE

-not all genes follow Mendel's laws

-incomplete dominance is when both alleles contribute to the phenotype

Japanese 4 o'clock flowers:

pure: red X white

-->all pink

hybrid: pink X pink

-->1 red:2 pink:1 white

CO-DOMINANCE

-2 different alleles are expressed at the same time

e.g. roan coat horses, both red and white hairs

-genotypes for co- dominance are expressed in superscript

pure:CWCW X CRCR

-->CRCW

MULTIPLE ALLELES

-refers to traits that have more than 2 alleles in a species, but not in the same individual

e.g. human blood groups

3 possible alleles

ABO groups

4 major human blood types

2 antigens

"A" and "B" found on surfaces of RBC determine blood type

"O" no antigen

antigen and antibody information critical for blood transfusions

All genes present expressed-no dominance/recessive

Possible combinations (2 alleles in each person, one from mother one from father)

IA IA type A blood

IB IB type B blood

IA IB type AB blood

IA i type A blood

IB i type B blood

ii type O blood

If you have type AB blood, we know your genotype

If you have type O blood we know your genotype

otherwise....

A father AB can not have a child O

Prior to DNA testing

blood type was used to narrow down paternity in legal cases.

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