Genetics

Mendel and the Gene Idea

Multifactorial Inheritance

Genotype does not rigidly define the phenotype, but a range of possibilities over which the environment can influence

genes + environment & diet phenotype

Phenotype depends on environment and genes

Tree: leaves that vary in size, shape, and greenness, depending on exposure to wind and sun.

Humans: nutrition influences height, exercise alters build, sun-tanning darkens the skin, and experience improves performance on intelligence tests.

Identical twins: genetic equals, accumulate phenotypic differences as a result of their unique experiences.

Environment contributes to the phenotype

acidic soil basic soil

Early Ideas about Heredity

Before Mendel basic facts of heredity studied garden pea plant formed hybrids found recessive traits in some offspring

Gregor Mendel

father of genetics mentors - physics, botany quantified results subject - garden pea plant

1843 – entered monastery 1851-53 – studied at U of Vienna 1857 – began breeding pea plants

Advantages of peas

Mendel had strict control over which plants mated with which

Each pea plant has male (stamens) and female (carpal) sexual organs.

Mendel could allow self-pollination or also move pollen from one plant to another to cross-pollinate

Mendel’s Seven Characters and Alleles

Mendel’s Experimental Design self-pollinate

crossed alternative traits to produce hybrids (hybrids expressed only dominant traits)

hybrids were self-pollinated

(produced 3:1 ratio)

Reginald Crundall Punnett

Punnett square

Symbols

P = parental generation F1 = first filial generation

F2 = second filial generation

Terminology

homozygous

homozygous dominant = RR homozygous recessive= rr

heterozygous = Rr genotype and phenotype complete dominance

Monohybrid Cross

parental varieties differ in a single character

character = flower color (using letter P)

allelles purple, dominant (PP, Pp) white, recessive (pp)

Flower Color (Pea plant)

topveg.com/wp-content/uploads/2009/03/pea-flower-ambience-300x286.jpgaustralianmajestictours.com/hovea.jpg

Monohybrid CrossF1 purple (PP) X

white (pp) dominant x

recessive F1 are all purple (Pp) all Pp = hybrid

Monohybrid Cross Ratio in F2 phenotypic ratio 3:1

3 dominant: 1 recessive PP & Pp are

indistinguishable in complete dominance

genotypic ratio 1:2:1 1 homozygous dominant (PP) 2 heterozygous (Pp) 1 homozygous recessive (pp)

Testcross (Pedigree Analysis) to determine unknown genotype utilizes a homozygous recessive phenotypic ratio of offspring determines genotype

F2 F1 P

Example

F2

75% are black (B_) 25% are brown (bb)

bb B_

B_ B_

B_ bb

B b

B BB Bb

b Bb bb

F1

all black (Bb)

B B

b

b Bb Bb

Bb Bb

Black Labrador (BB) Brown Labrador (bb)

Parent generation (P)

Mendel’s First Law"Law of Segregation"

allele pairs separate during gametogenesis then randomly reform pairs at fertilization

Dihybrid Cross

parental varieties differ in two characters characters: seed color (Y) seed shape (R)

seed color alleles yellow: dominant (YY, Yy) green: recessive (yy)

seed shape alleles round: dominant (RR, Rr) wrinkled: recessive (rr)

P: yellow/round X green/wrinkled F1 : dihybrid (YyRr)

F2 : offspring (9:3:3:1)

9 Y_R_

3 Y_rr

3 yyR_

1 yyrr

P1

F1

F2

How do I know my Punnett is done correctly? look at the diagonals!

all four are heterozygous on both genes (YyRr)

all four are homozygous on both genes

Mendel’s Second Law"Law of Independent Assortment"

each allele pair assorts independently from one another during gamete formation

Patterns of Inheritance not described by Mendel still follow Mendel’s laws

Incomplete dominance Codominance Pleiotropy Epistasis Polygenic inheritance

Incomplete Dominance

Blends are formed examples:

Snapdragons Tay-Sachs disease

Snapdragon

Intermediate phenotype Red = CRCR Pink = CRCW White = CWCW

Codominance

Both alleles are expressed examples:

Landsteiner blood groups (A x B = AB)

shorthorn cattle (white x red = roan)

calico cat (orange-yellow x black)

Blood groups: Codominance & multiple alleles

http://health.stateuniversity.com/article_images/gem_01_img0116.jpg

Blood type Testcoagulates when it has the

antibody for that type of blood

same happens with the Rh group

Blood Genotypes

IAIA homozygous IAi heterozygous IBIB homozygous IB i heterozygous IAIB

ii Rh group functions independently

by complete dominance rules: DD or Dd is Rh positive dd is Rh negative

Blood type A

Blood type B

Blood type AB

Blood type O

Examples : IAiDd A+ IBIBdd B- iiDD O+

Class Activity

A X B = O is it possible? What are the genotypes of the parents?

AB X O = O is it possible? Give the genotypes of the possible

children

Shorthorn Cattle Codominance

RedWhite

Roan

Calico Cat Codominance

B = black R = red XBXR

black XB

red XR

Barr body

Barr body is an inactive strand of DNA

Pleiotropy

single gene has multiple effects example:

in Siamese cats, one gene causes abnormal fur pigmentation and esophoria (crossed-eyed)

Waardenburg syndrome white forelock pale iris deafness

http://dermatology.cdlib.org/123/case_presentations/waardenburg/1.jpg

Epistasis

one gene alters the expression of another that is independently inherited example:

albinism in animals (mice) 9 B_ C_ = black 3 B_ cc = albino 3 bb C_ = brown 1 bb cc = albino 9:3:4 instead of 9:3:3:1

An additive effect of 2 or more genes on one character

example skin pigmentation

at least 3 genes hair color

at least 4 genes

Polygenic Inheritance

Rule of Addition

The probability of an event that can occur in 2 or more different ways is the sum of the separate probabilities of those ways

Rule of multiplication Multiply individual probabilities to get

overall probability

Rules of Addition and Multiplication Female Male

roll tongue (Rr) no roll tongue (rr)

long eyelashes (Ll) short eyelashes (ll)

cleft chin (Cc) cleft chin (Cc)

Probability of having a daughter 2/4 (xx, xx, xy, xy) roll tongue 2/4 (Rr, Rr, rr, rr) long eyelashes 2/4 (Ll, Ll, ll, ll) cleft chin 3/4 (CC, Cc, Cc, cc)

2/4 x 2/4 x 2/4 x 3/4 = 24/256 = 9.375%

Activity

Eye color dark/light P_/ppTongue rolling yes/no R_/rrsmall finger crooked/straight B_/bbwidow's peak yes/no W_/wwhand clasping right/left C_/ccearlobe free/attached A_/aaHitchhiker's thumbno/yes H_/hhPTC tasting yes/no D_/dd

The End