Mendel, Genes, and Inheritance

Chapter 12

Gregor Mendel Austrian Monk with a strong background in plant

breeding and mathematics Using pea plants, found indirect but observable

evidence of how parents transmit genes to offspring

Mendel’s Work Bred pea plants

cross-pollinated true breeding parents (P)raised seed & then

observed traits (F1) filial

allowed offspring to cross-pollinate & observed next generation (F2)

Mendel’s Data:

Mendel’s Experimentstrue-breeding

purple-flower peastrue-breeding

white-flower peasX

100%100%

purple-flower peasF1generation(hybrids)

25%white-flower peas

F2generation

75%purple-flower peas

3:1

P

self-pollinate

Genes Sequences of DNA that contain

information about specific traits, by coding for individual proteins

Passed from parents to offspring Each has a specific location (locus) on a

chromosome

Alleles Different versions of the same trait

Genes and inheritence

For each characteristic (gene), a diploid organism inherits 2 sets of alleles1 from each parent

Homologous chromosomesPairs of chromosomes that contain the same

traits (genes)For each trait there are two alleles (one on

each homologous chromosome)

Genetic Terms

purple-flower allele & white-flower allele are 2 DNA variations at flower-color locus

different versions of gene on homologous chromosomes

Mendel’s Findings: Dominance

Some alleles for a given traits mask others Dominant allele

Fully expressed in a hybridDesignated by a capital letter

e.g. P = Purple allele

Recessive allele no noticeable effect in a hybridDesignated by a lowercase letter

E.g. p = White allele

Genotype vs. phenotype Phenotype

Description of an organism’s trait“visible” characteristic

Genotype Description of an organism’s genetic makeup;

i.e. which alleles are present in the organismAlleles may be the same or they may be

different:

Homozygous = same alleles; PP, pp Heterozygous = different alleles; Pp

Looking closer at Mendel’s worktrue-breeding

purple-flower peastrue-breeding

white-flower peasX

100%purple-flower peas

25%white-flower peas

75%purple-flower peas

3:1

PP pp

Pp Pp Pp Ppself-pollinate

????

Phenotype:

100%

100%purple-flower peasF1

generation(hybrids)

F2generation

75%purple-flower peas

3:1

P

Punnett squares

Pp x Pp

P pmale / sperm

P

p

fem

ale

/ eg

gs

PP

Pp pp

Pp

PP

Pp

Pp

pp

75%

25%

3:1

25%

50%

25%

1:2:1

%genotype

%phenotype

Phenotype vs. genotype

2 organisms can have the same phenotype but have different genotypes

PP homozygous dominantpurple

Pp heterozygouspurple

Dominant phenotypes It is not possible to determine the

genotype of an organism with a dominant phenotype by looking at it.

So how can you figure it out?

PP?

Pp?

Test cross

Cross-breed the dominant phenotype — unknown genotype — with a homozygous recessive (pp) to determine the identity of the unknown allele

x

ppis itPP or Pp?

x

PP pp

Test cross

p p

P

P

Pp

Pp Pp

Pp

p p

P

p

Pp

pp pp

Pp

x

Pp pp

100% 50%:50%1:1

Mendel’s laws of heredity (#1)

PP

P

P

pp

p

p

Pp

P

p

Law of segregation when gametes are produced

during meiosis, homologous chromosomes separate from each other

each allele for a trait is packaged into a separate gamete

Law of Segregation

What meiotic event creates the law of segregation?

Meiosis 1

Monohybrid cross Some of Mendel’s experiments followed the

inheritance of single characters flower colorseed color monohybrid crosses

Dihybrid cross

Other of Mendel’s experiments followed the inheritance of 2 different characters seed color and

seed shape Dihybrid crosses

Dihybrid crosstrue-breeding

yellow, round peastrue-breeding

green, wrinkled peasx

YYRR yyrrP

YyRr

100%F1generation(hybrids)

yellow, round peas

self-pollinate

F2generation

9/16yellowround peas

9:3:3:13/16

greenround peas

3/16yellow

wrinkledpeas

1/16green

wrinkledpeas

Y = yellowR = round

y = greenr = wrinkled

Dihybrid Cross How are the alleles on different chromosomes

handed out? together or separately?

YyRr

YR yr

YyRr

Yr yRYR yr

Dihybrid crossYyRr YyRr

YR Yr yR yr

YR

Yr

yR

yr

YYRR YYRr YyRR YyRr

YYRr YYrr YyRr Yyrr

YyRR YyRr yyRR yyRr

YyRr Yyrr yyRr yyrr

x9/16

yellowround

3/16greenround

3/16yellow

wrinkled

1/16green

wrinkled

Mendel’s laws of heredity (#2)Law of independent assortment: each pair of alleles segregates into gametes

independently 4 classes of gametes are produced

in equal amounts YR, Yr, yR, yr

only true for genes on separate chromosomes

YyRr

Yr Yr yR yR YR YR yr yr

Law of Independent Assortment What meiotic

event creates the law of independent assortment?

Meiosis 1

The chromosomal

basis of Mendel’s laws…

Trace the genetic events through meiosis, gamete formation & fertilization to offspring

Review: Mendel’s laws of heredity Law of segregation

each allele segregates into separate gametes Law of independent assortment

Observable in dihybrid (or more) cross 2 or more traits

each pair of alleles for genes on separate chromosomes segregates into gametes independently

Each gamete carries one allele of each trait

Extending Mendelian genetics Mendel worked with a simple system

peas are genetically simplemost traits are controlled by a

single geneeach gene has only 2 alleles, 1 of which is

completely dominant to the other The relationship between

genotype & phenotype is rarely that simple

Incomplete dominance Heterozygotes show an intermediate

phenotypeRR = red flowersrr = white flowersRr = pink flowers

make 50% less color

Incomplete dominancetrue-breeding

red flowerstrue-breeding white flowers

X

100%

100% pink flowers

F1generation(hybrids)

25%white

F2generation

25%red 1:2:1

P

self-pollinate50%pink

Incomplete dominance

CRCW x CRCW

CR CW

male / sperm

CR

CW

fem

ale

/ eg

gs CRCR

CRCW CWCW

CRCW

25%

1:2:1

25%

50%

25%

1:2:1

%genotype

%phenotype

CRCR

CRCW

CRCW

CWCW

25%

50%

Codominance

BOTH alleles express in heterozygotes Example: In chickens, feather color trait

has two alleles:B= Black feathers

W = White feathers

What is phenotype of BB, WW?What is the phenotype of BW?

Codominance:

WW BB

WB

+

Multiple Alleles

More than two possible alleles for one trait Example: in ABO blood group, 3 possible

alleles: IA, IB, i

4 blood types: A, B, O, AB How?

Pleiotropy Most genes are pleiotropic

one gene affects more than one phenotypic character

wide-ranging effects due to a single gene: dwarfism (achondroplasia) gigantism (acromegaly)

Epistasis One gene masks another

coat color in mice = 2 genes

pigment (C) or no pigment (c)

more pigment (black=B) or less (brown=b)

cc = albino, no matter B allele

9:3:3:1 becomes 9:3:4

Epistasis in Labrador retrievers 2 genes: E & B

pigment (E) or no pigment (e) how dark pigment will be: black (B) to brown (b)

Polygenic inheritance (continuous variation)

Some phenotypes determined by additive effects of 2 or more genes on a single characterphenotypes on a continuumhuman traits

skin color height weight eye color intelligence behaviors

Environmental influence

The expression of some genes can be influenced by the environment

for example: coat color in Himalayan rabbits and Siamese catsan allele produces an enzyme that allows

pigment production only at temperatures below 30oC

Environmental influence

Acknowledgement

Much of this presentation was modified from the Biology Zone website:

http://bio.kimunity.com/