MENDEL AND THE

GENE IDEA

CHAPTER 14

GREGOR MENDEL

Austrian monk

Brought experimental and

quantitative approach to

genetics

Bred pea plants to study

inhertance

Why peas?

Control mating (self- vs. cross-

pollination)

Many varieties available

Short generation time

P (parental) generation = true breeding plants

F1 (first filial) generation = offspring

F2 (second filial) generation = F1 offspring

7 CHARACTERS IN

PEA PLANTS

(OBSERVED 3:1

RATIOS)

Dominant vs. Recessive

(expressed) or (hidden)

FIRST- ALTERNATIVE VERSIONS OF GENES ACCOUNTS

FOR VARIATIONS IN INHERITED TRAITS

ALLELES: ALTERNATE VERSIONS OF A GENE

SECOND- AN ORGANISM INHERITS TWO

ALLELES, ONE FROM EACH PARENT. A

GENETIC LOCUS IS REPRESENTED TWICE

THIRD-IF TWO ALLELES AT A LOCUS DIFFER

THEN THE DOMINANT ALLELE DETERMINES THE

ORGANISMS APPEARANCE

dominant (P), recessive (p)

homozygous = 2 same alleles (PP or pp)l; fully

epxressed

heterozygous = 2 different alleles (Pp); no

noticeable effect; gene make a non-functional

protein

FOURTH-LAW

OF

SEGREGATION

The two alleles for a

heritable character

separate (segregate)

during gamete formation

and end up in different

games

MENDEL’S PRINCIPLES

1. Alternate version of genes (alleles) cause variations in inherited characteristics among offspring.

2. For each character, every organism inherits one allele from each parent.

3. If 2 alleles are different, the dominant allele will be fully expressed; the recessive allele will have no noticeable effect on offspring’s appearance.

4. Law of Segregation: the 2 alleles for each character separate during gamete formation.

Law of Independent Assortment:

Each pair of alleles segregates (separates)

independently during gamete formation

Eg. color is separate from shape

Testcross: determine if dominant trait is

homozygous or heterozygous by crossing with

recessive (pp)

Phenotype: expressed physical traits

Genotype: genetic make-up

PUNNETT SQUARE Device for predicting offspring from a cross

Example: Bb x Bb (B=purple, b=white)

Genotypic Ratio: 1 BB, 2 Bb, 1 bb

Phenotypic Ratio: 75% or 3/4 purple and 25 % or ¼ white

Monohybrid cross: study 1 character

eg. flower color

Dihybrid cross: study 2 characters

eg. flower color & seed shape

DIHYBRID CROSS (VIDEO)

Example: AaBb x AaBb

F: Take the FIRST allele of each gene (left eyebrow) to make

one possible gamete.

O: Take the OUTER alleles (big smile) to make one possible

gamete.

I: Take the INNER alleles (nose curve) to make one possible

gamete.

L: Take the LAST allele of each gene (right eyebrow) to make

one possible gamete.

SEGREGATION OF ALLELES AND

FERTILIZATION AS CHANCE EVENTS

Probability Rules:

a) AND= multiply

½ x ½ = ¼

a) OR=add

¼ + ¼= 2/4 =1/2

THE LAWS OF PROBABILITY GOVERN MENDELIAN

INHERITANCE

Rule of Multiplication:

Probability that 2+ independent events will occur together in a

specific combination multiply probabilities of each event

Ex. 1: probability of throwing 2 sixes

1/6 x 1/6 = 1/36

Ex. 2: probability of having 5 boys in a row

½ x ½ x ½ x ½ x ½ = 1/32

Rule of Addition:

Probability that 2+ mutually exclusive events will occur add

together individual probabilities

Ex. 1: chances of throwing a die that will land on 4 or 5?

1/6 + 1/6 = 1/3

EXTENDING MENDELIAN GENETICS The relationship between genotype and phenotype

is rarely simple

Complete Dominance: heterozygote and

homozygote for dominant allele are

indistinguishable

• Eg. YY or Yy = yellow seed

Incomplete Dominance: F1 hybrids have

appearance that is between that of 2 parents

• Eg. red x white = pink flowers

Codominance: phenotype of both alleles is

expressed

• Eg. red hair x white hairs = roan horses

• A dominant allele does not subdue a recessive allele; alleles

don’t interact

• Alleles are simply variations in a gene’s nucleotide

sequence

• For any character, dominance/recessiveness relationships of

alleles depend on the level at which we examine the

phenotype

Tay-Sachs disease is fatal; a dysfunctional enzyme causes an

accumulation of lipids in the brain

At the organismal level, the allele is recessive

At the biochemical level, the phenotype (i.e., the enzyme activity level) is incompletely dominant

At the molecular level, the alleles are codominant

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

The Relation Between Dominance and Phenotype

BLOOD TYPING

Multiple Alleles: gene has 2+ alleles

• Eg. human ABO blood groups

• Alleles = IA, IB, i

• IA,IB = Codominant

http://www.youtube

.com/watch?v=KX

TF7WehgM8

11 min

BLOOD TYPING

Phenotype

(Blood Group) Genotype(s)

Type A IAIA or IAi

Type B IBIB or IBi

Type AB IAIB

Type O ii

BLOOD TRANSFUSIONS

Blood transfusions must match blood type

Mixing of foreign blood clumping death

Rh factor: protein found on RBC’s (Rh+ = has

protein, Rh- = no protein)

Pleiotropy: single gene has multiple phenotypic

effects (Eg. sickle cell anemia, cystic fibrosis,

dwarfism)

Epistasis: one gene alters the phenotypic

expression of another gene (eg. albinism - white

fur color in mammals)

B- black B- brown E- dep. of pigment E- no dep. of pigment ee overrides the expression of all the B,b genotypes 9:3:4

Polygenic Inheritance: the effect of 2 or more

genes acting upon a single phenotypic character

(eg. skin color, height, weight, eye color)

Nature and Nurture: both genetic and

environmental factors influence phenotype

Hydrangea flowers vary in shade and intensity of color

depending on acidity and aluminum content of the soil.

GENETIC SCREENING AND COUNSELING

Based on Mendelian genetics and probability

rules

Tests for identifying carriers

Fetal testing

Newborn Screening

GENETIC TESTING

Amniocentesis

- Cant be done until 14 to 16

weeks

- Needle inserted through

abdomen

- Sample of amniotic fluid

removed

- Biochemical tests done

immediately or cultured

- Takes weeks, risk of

miscarriage

Chorionic Villus

Sampling (CVS)

- Can be done 8 to 10 weeks

- Suction tube inserted through

cervix

- Biochemical tests/karyotyping

can be done immediately

- Faster results

- Higher chance of miscarriage

PEDIGREE ANALYSIS

Pedigree: diagram that shows the relationship between parents/offspring

across 2+ generations; follows transmission of traits over generations