Heredity

Non-Mendelian Genetics

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Mendelian Heredity

• Principle of Dominance 

• Law of Segregation

• Law of Independent Assortment

http://mac122.icu.ac.jp/gen-ed/mendel-gifs/01-mendel-himself.JPG

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Non-Mendelian Heredity

• does not fit Mendel’s Laws

• is not governed by one autosomal gene

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Incomplete Dominance

• Heterozygotes look different from a homozygotes

• Blending of phenotypes

http://chsweb.lr.k12.nj.us/psidelsky/Chapter10(Gen).htm

RR x RRIIRRII

RRI

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Codominance: Both alleles of a gene pair are expressed.

Example: Human Blood Types• Human blood type is determined by codominant

alleles. There are three different alleles: IA, IB, and i. The IA and IB alleles are codominant, and the i allele is recessive.

• The possible human phenotypes for blood group are type A, type B, type AB, and type O.

• Type A and B individuals can be either homozygous (IAIA or IBIB, respectively), or heterozygous (IAi or IBi, respectively). Type O is IOIO.

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Codominance: More Examples

Roan coloration in cows (red coat with white

blotches)

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http://upload.wikimedia.org/wikipedia/commons/f/f3/Co-dominance_Rhododendron.jpg

http://en.wikipedia.org/wiki/File:Light_Roan_Shorthorn_Heifer_DSCN1872b.jpg

Rhododendron coloration

THINK-PAIR-SHARE

1. Distinguish between incomplete dominance and codominance.

2. How many phenotypes are seen when codominance is possible?

3. Red color (R) in snapdragons shows incomplete dominance to white snapdragons (RI). The heterozygotes are pink. If pink snap dragons are crossed with a white snapdragon, what are the genotypic/ phenotypic ratios?

R RI

RI RRI RIRI

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Multiple Alleles

• When 4 or more possible phenotypes exist within a population– Blood type– Eye & hair color– Fur in mammals

http://www.learner.org/interactives/dna/genetics8.html

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Problems with Multiple Alleles

A woman with type O blood marries a man with type AB blood. What are the possible blood types that a child of theirs could inherit?

STEP 1: ♀ ii ♂ IAIB

STEP 2: i IA , IB

STEP 3:

STEP 4:

STEP 5:

STEP 6:

IA IB

IAi IBii

There’s a 50% chance that their baby will be Type A or Type B.

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Polygenic Inheritance

Poly-many genic-gene• More than one gene

on more than one loci or on more than one chromosome.

• Give the appearance of gradation – Hair & skin color– Height– Body size

http://scienceblogs.com/geneticfuture/a_ruler_beats_genetics_in_pred/tall_short1.jpg

http://www.nature.com/nrg/journal/v10/n12/box/nrg2670_BX1.html

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THINK-PAIR-SHARE

Black fur (BBl) is completely dominant to brown fur in rabbits, but brown fur (BBr) is dominant to white (b). A hybrid brown doe (BBrb) is crossed with a hybrid black buck (BBlb). What are the expected phenotypes of the offspring? Ratios?

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Linked Traits

• When the loci of two genes are located close together on the same chromosome– Red hair, freckles, and

fair skin are linked.

http://en.wikipedia.org/wiki/File:Woman_redhead_natural_portrait_1.jpg

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Sex-Linked Traits

http://www.ccs.k12.in.us/chsteachers/Amayhew/Biology%20Notes/sexlinked%20notes.htm

• Genes located on a sex (X or Y) chromosome

♀XX & ♂XY – Some forms of

hemophilia– Red-green

colorblindness

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Red-Green Colorblindness

http://upload.wikimedia.org/wikipedia/commons/e/e0/Ishihara_9.png

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Hemophilia

http://biology.clc.uc.edu/courses/bio105/sex-link.htm

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Barr Bodies

• The inactive X chromosome found in female (XX) somatic cells– Not found in normal

males (XY) .– Explains calico

coloring in cats.http://www.genetics.com.au/factsheet/fs14.asp

The nucleus of a cell. The arrow is pointing to the inactive X

chromosome, the Barr Body.

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Calico Cats

http://waynesword.palomar.edu/biex4hnt.htm

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Solving Sex-Linked Problems

A woman who is a carrier for red-green colorblindness marries a man with normal sight. What percent of the male and female children will be red-green colorblind or carriers?

STEP 1: ♀ XX' ♂ XY

STEP 2: X, X' X, Y

STEP 3:

STEP 4:

STEP 5:

STEP 6:

X Y

XX XYX

X' XX' X'Y

50% ♀ Normal

50% ♀Carriers

50% ♂Normal

50% ♂ Colorblind

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THINK-PAIR-SHARE

1. Is the trait sex-linked? How do you know?

2. How could we tell if female #5 is heterozygous if she never has children?

3. What are the expected phenotypic ratios from a cross between #3 & #7?

http://upload.wikimedia.org/wikipedia/commons/c/c5/Autosomal_Recessive_Pedigree_Chart_.svg

1 2

3 4 5 67 8

Wild Type

Homozygous

Heterozygous

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