Biology

Chapter 9

Fundamentals of Genetics

What is Genetics?

a. Study of heredityb. Transmission of traits from parent to

offspring

Who is the father of genetics?

• Gregor Mendel

• 1822-1884

Why was Mendel Successful?

• 2 reasons1. He used a garden pea as his test subject

• Why would he use the pea?• 6 reasons

a. Smallb. Easy to growc. Produce many offspringd. Mature quicklye. Many varietiesf. Easy to fertilize

» Self fertilization – within same plant» Cross fertilization – involved two plants

Second reason?

2. He used a quantitative approach

Mendel’s Experiment

• Step 1– He produced a parent generation (P)

• He allowed pea plants to self fertilize for many generations

• This made sure that he had pure parents that were true breeding or pure

• Example: He has 1 pure purple pea plant and 1 pure white pea plant for his P generation

Mendel’s Experiment Cont.

• Step 2– He produced the 1st generation (F1)– He cross fertilized two of the P generation pea

plants– What was his results?

• 100% Purple plants, no white

Mendel’s Experiment Cont.

• Step 3– He produced the 2nd generation (F2)– He took 2 of the F1 generation pea plants and

self fertilized them.– What were the results?

• 75% purple, and 25% white

Mendel’s Conclusions

• Parents transmit information about traits to their offspring

• Each individual has 2 factors (genes) for each trait, 1 from each parent

• Factors (genes) are represented by letters or alleles.

Alleles

• If both alleles are the same, the individual is homozygous for that trait

• If both alleles are different, the individual is heterozygous for that trait

Traits can be described in 2 ways:

• Genotype– Alleles that represent the trait– Example: PP, Pp, pp

• Phenotype– Expression of the trait– Example: purple, white

More about traits

• Only some traits are seen, others are masked– Dominant – only need 1 letter to be expressed

» Represented by capital letters

– Recessive – need both letters or its masked» Represented by lowercase letters

Do you understand?Genotype Dom/Rec Homo/Hetero Pheno

Mendel’s Law of segregation

• Members of each pair of alleles separate when gametes are formed. A gamete will receive 1 allele of the other. This occurs in meiosis.

Principle of Independent Assortment

• Two or more pairs of chromosomes separate independently of one another during the formation of gametes. This is random.

Why did Mendel’s results repeat?1. Chance and probability- Leads to predictions

1. coins- flipping a head?

2. cards- diamond?- nine?- nine of diamonds?

3. sex of children- having a boy child?- having a girl after having 4 boys in a row?

Monohybrid Crosses

• Involves 1 trait

• Crosses 2 alleles on the same locus

• Uses a 4-boxed Punnett Square

• Example: Cross a white flowered pea plant with a heterozygous purple flowered pea plant

Monohybrid Example

• pp x Ppp p

P

p

Genotype %

50% Pp

50% pp

Phenotype %

50% purple

50% white

How do you find out whether an individual is BB or Bb?

• To a testcross– Technique that takes the unknown genotype and cross it

with a recessive individual and then look at the results.

Dihybrid Crosses

• Involves 2 traits• Crosses individuals with 4 alleles at 2 loci• Uses a 16 box punnett square• When both genotypes for both individuals are

heterozygous (BbTt x BbTt), the phenotype percentage will be 9:3:3:1

• Example: Cross a homozygous purple flowered, heterozygous green pod pea plant with a white flowered, yellow pod pea plant. (Green is dominant over yellow)

Dihybrid Example

• PPGg x ppgg• Must do Foil to get the gametes!

– First, Outer, Inner, Last• PPGg• F = PG• O = Pg• I = PG• L = Pg

– FOIL for ppgg are pg, pg, pg and pg– So now take these gametes and place them

in the Punnett Square

Dihybrid ExampleDihybrid Example

Genotype: 50% PpGg, 50% Ppgg Phenotype: 50% Purple Green

50% Purple Yellow

Dominant Recessive relationships

1. Lethal recessive – homozygous recessive organisms cannot survive (ex. Tay Sachs, Cystic Fibrosis)

2. Incomplete Dominance

3. Codominance

Incomplete Dominance

• Heterozygote is an intermediate between phenotypes of two homozygotes

• Blending occurs!• Occurs in Japanese 4:00 plants and

snapdragons– RR = red– WW = white– So RW = pink!

• Example: Cross a red flowered Japanese 4:00 plant with a white flowered 4:00 plant

Incomplete Dominance Example

Genotype:

100% RW

Phenotype:

100% pink

Codominance

• Two traits share dominance (Ex. Human Blood Types)– Must use special notations when doing these

problems• IAIA and IAi…………bloodtype A• IBIB and IBi…………bloodtype B• IAIB………………….blood type AB• ii……………………..bloodtype O

• Cross a person with bloodtype AB with a person with bloodtype O

Codominance Example

IAIB x ii IA IB

i

i

IAi IBi

IAi IBi

Genotype %

50% IAi

50% IBi

Phenotype %

50% bloodtype A

50% bloodtype B

X linked Genes

• Also called Sex linked genes

• Genes that follow the transmission of the X chromosome

• Always expressed in males, and is considered to be dominant

• Females may be expressed

• Hemophilia is an example – special notation for these types of problems

Sex linked notations

• For hemophilia– Females

• XHXH normal• XHXh carrier/heterozygous• XhXh has hemophilia

– Males• XHY normal• XhY has hemophilia

Example: Cross a hemophiliac man with a female carrier

Sex Linked ExampleXHXh x XhY Xh

Xh

XH

Y

XHXh

Genotype

25% XHXhXHY

25% XHYXhXh XhY 25%XhXh

25% XhY Phenotype: 50% normal, 50% hemophiliac