Chapter 8 Mendel & Heredity

I. Terms A. Heredity- the passing on of traits (color/shape

of eyes, texture of hair) from parent to offspring

B. Genetics- the study of heredity

C. Genes- pieces of DNA that code for a certain trait (Mid-digital hair)

D. Alleles- alternative versions for each gene

(ex. hairy mid-digits, non hairy mid-digits)

1) Letters are used to represent alleles (Capital letters represent dominant trait, lower case represent recessive trait)

Terms continuedE. Homozygous- 2 alleles in an individual

are the same (ex. HH, hh), pure bred

F. Heterozygous- alleles are different (ex. Hh), hybrid

G. Genotype- the set of alleles that an individual has (ex. HH, Hh, hh)

H. Phenotype- the physical appearance of a trait (ex. Hairy mid-digits, non hairy mid digits)

II. Gregor Mendel

A. Gregor Mendel (1800’s) - Father of Genetics p.161

B. http://science.discovery.com/videos/100-greatest-discoveries-shorts-genetics.html

Mendel

B. Studied 7 traits in garden peas (grows quickly & produces many offspring)

• ***Traits- pod color, flower color, wrinkled, height, seed shape, etc.

• -Self-pollination produced the same traits every time

• Ex. Purple x Purple = All Purple flowers White x White = All White

• Crossed 2 plants with contrasting traits (P1 x P1 = F1)

– Ex. Pure white x Pure purple = All purple plants F1 (filial) generation

• 2nd cross between plants of F1 generation yielded the F2 generation– F1(purple) x F1 (purple) = F2 (75% purple & 25%

white)

• *(Before Mendel, people thought offspring were a blend of their parents Ex. Tall x Short = Medium height)

C. Mendelian theory of heredity

1. For each trait, an individual has two copies of the gene—one from each parent

2. There are alternative versions of genes (alleles)

3. The allele that is displayed is called dominant; the allele that is present in the organism but has no effect on its appearance is called recessive

1.Law of Segregation= the two alleles for the trait separate when gametes are formed. Gametes carry only one allele for each inherited trait

2. Law of independent assortment= the alleles of different genes separate independently of one another when gametes are formed.

III.Punnett square

A. Tool used by scientist to predict the outcome of a genetic cross

• Monohybrid sample problem: • In roses, red is dominant over white. • Use R for the red gene. • Use r for the white gene. • Cross two heterozygous red roses,

Rr X Rr • From the Punnett Square,

– describe the phenotype of the offspring.

• 3/4 Red • 1/4 White

– describe the genotype of the offspring.

• 1/4 RR • 1/4 rr • 2/4 (or ½) Rr

• </TD

IV. Beyond Complete Dominance

A. Incomplete dominance- when neither allele is dominant over the other; results in blending the heterozygous genotype (Rr)

Examples:

Incomplete Dominance:

#1. One trait in humans that is displayed by incomplete dominance is hair texture. The heterozygous genotype (Hh) is expressed as wavy. If mom has curly hair, which is dominant to straight hair, and dad has wavy hair then what is the probability of them having an offspring that has curly hair?

Can they have an offspring that has straight hair?

B. Co-dominance- both alleles are expressed in the heterozygote equally

Ex. Multiple alleles inheritance (blood type)

Example

Co-dominance

#2. Roan cattle are a result of co-dominance. List the possible phenotypes of the offspring that would result from the crossing of 2 roan cattle.

C. Sex-linked inheritance- traits carried on the X chromosome

ex. color blindness (red/green),

hemophilia (blood clotting disorder)

Punnett Square Examples:

XX=female

XY=male

More Examples of Sex-Linked Recessive Disorders

• Red/Green Colorblindness – Difficulty perceiving differences between colors (red or green, blue or yellow).

• Hemophilia – Absence of one or more proteins necessary for normal blood clotting.

• Deafness• Cataracts – opacity in the lens that can lead to blindness• Night blindness – (Nyctalopia) rods do not work so that can not

see in the dark• Glaucoma – pressure in the eye that can lead to optic nerve

damage and blindness• Duchenne Muscular Dystrophy – progressive weakness and

degeneration of skeletal muscles that control movement due to absence of dystrophin (protein that maintains muscle integrity). Mainly in boys, onset 3-5 yrs, by 12 years can’t walk, and later needs respirator.

Example

Sex-linked disorders:

#3. Hemophilia is a blood clotting disorder that is sex-linked recessive disorder (found on the X chromosome). Demonstrate a cross between a mom who is a carrier for the disorder and a dad who does not have hemophilia.a) What percent of the offspring will have hemophilia?b) What percent of the females are carriers?c) What percent of the males have hemophilia?

Example

Sex Linked Inheritance & Co-dominance:#4. Coat color in cats is a co-dominant trait

and is also located on the X chromosome. Cats can be black, yellow or calico. A calico cat has black and yellow splotches. In order to be calico, the cat must have an allele for the black color and an allele for the yellow color.

#4. A female calico cat is crossed with a male black cat.

a) What percent of the offspring are calico?

b) What percent of the females are calico?