4. mendelian inheritance

8/11/2019 4. Mendelian Inheritance

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Principles of Mendelian

Inheritance


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Gregor Mendel

Experiments in Plant

Hybrids (1866)

particulateinheritance

1822-1884


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Seven Characteristics

Studied by Mendel

Form of ripe seed

Smooth or wrinkled Color of seed albumen

Yellow or green

Color of seed coat

Gray or white

Form of ripe pods

Inflated or constricted

Color of unripe pods Green or yellow

Positions of flowers on

stems

Axial or terminal

Length of stem

Tall or short


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Carefully controlled experiments

(minimal chance of accidental

pollination) with large samplesizes (28,000 plants over 8 years);

either/or expression traits


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1. Principle of Segregation Different expressions of a trait are controlled by

discrete particles that occur in pairs

Offspring inherit one particle from each parent no blending

During fertilization the particles reunite todetermine the expression of the trait in the

offspring.


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Allele, Gene, and Locus

GENEsegment of DNA that directs the

production of a specific protein

ALLELE- Alternate forms of a gene that

can direct the cell to produce slightly

different forms of the same protein and

ultimately, different expressions of traits

LOCUSlocation of a gene on a

chromosome


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Heterozygous and Homozygous

Homozygous

Two copies of the

same allele present at

a locus

For example, TT

Heterozygous

Two different alleles at

a locus

For example, Tt


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Genotype and Phenotype

Genotype

The specific genetic

makeup of a trait

Represented by letters

For example, Tt

Phenotype

The physical

expression of the

genes, or what we see

For example, tall or

short plant


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2. Dominance and

Recessiveness Some traits are masked and therefore not

expressed.

Dominant: a trait governed by an allele that isexpressed in the presence of another, different

allele.

Recessive: a trait that is not expressed in thepresence of another allele.

There must be two alleles of the same recessive trait

to express that trait.


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F1Generation

F2Generation

Yellowparents

Green

parents

Allyellow

F0GenerationMendels Pea Plant

Experiments

showed that in theF2Generations there wasa 3:1 phenotypic ratio.

Yellow seeds are dominant


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F1Generation

F2Generation

Yellow

parents

Green

parents

Allyellow

F0Generation

yy YYx

Yy

Yy Yy YYyy

Hybrid cross

Self-fertilizeYellow seeds are dominant


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yy

Y

Yy Yy

Y

Y

Y

Male gametes

Fe

malegametes

F0

Generation

100% of F1generation

plants are yellow

Yy = yellow

y y

Punnett Square/Mating Matrix


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3. Law of Independent

Assortment

Traits governed by different systems of

inheritance, e.g., smooth pea pods and seed

color, show no pattern of interdependence.

In other words, alleles for different traits sortindependently in the F1and subsequent

generations.


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LinkageAn exception to Independent Assortment

Genes whose loci are close together, on

the same chromosome, are often linked

(inherited together)


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F1Generation

F2Generation

Wrinkled,

greenparents

Smooth,

yellowparents

Allsmooth,yellow

F0Generation

Yellow is dominant YY

SSYY ssyyx

SsYy

Smooth,green

Wrinkled,yellow

Wrinkled,green

Smooth,yellow

Hybrid cross

Self-fertilize

Smooth is dominant SS

9 3 3 1


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sySY

Male gametes

Femalegamete

s

F1GenerationSelf-fertilizationSy sY

sy

SY

Sy

sY

SSYY SSYy SsYY SsYy

SSYy

SsYY SsYy ssYY ssYy

SsYy Ssyy ssYy ssyy

SSyy SsYy Ssyy

Yellow is dominant

Smooth is dominant

Smooth, yellow 9

Smooth, green 3

Wrinkled, yellow 3

Wrinkled, green 1

Phenotypic ratio 9:3:3:1


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Genetics Problem

AA Aa

Aa aa

A

a

a

A

The probability of the woman having normal height

is 25%, because the phenotypic ratio is:

3 abnormal : 1 normal, or 3/4


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Mendelian Inheritance in Humans

Monogenic vs.Polygenic

Online Mendelian

Inheritance in Man

http://www.ncbi.nlm.nih.gov/Omim/
http://www.ncbi.nlm.nih.gov/Omim/http://www.ncbi.nlm.nih.gov/Omim/http://www.ncbi.nlm.nih.gov/Omim/http://www.ncbi.nlm.nih.gov/Omim/


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ABO blood system

Governed by ABO locus

on chromosome 9

Alleles determine which

antigens are present onblood cells

Genotype Phenotype

AO, AA A

BO, BB B

AB AB

OO O

Codominant


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Common misconceptions

Eye color

Recessive alleles arecompletely hidden

Dominant alleles are mostcommon


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Tongue folding

Darwins tubercle


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Pedigree analysis


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Modes of Mendelian Inheritance

1. Autosomal dominant

2. Autosomal recessive

3. X-linked recessive4. X-linked dominant

5. Y-linked

6. Mitochondrial

} Sex-linked


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Autosomal dominant

Governed by (carried on) autosomesno

sex bias

If you inherit just one copy of the allele, the

trait will be expressed

Aa = dominant achondroplasia allele (A) is

expressed over the recessive normal (a)

allele


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Short stature, torso & head normal in

size, but

Short limbs, lordosis

Carried on Chromosome pair 4

QUESTION: Are these brothers from

one zygote? (identical or fraternal

twins?)


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The dwarf, Seneb,

and his family,

c. 2600 BC.

What is his genotype?

What genotype are

his children?

WHY?


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Autosomal recessive

MUST be homozygousto be affected

Heterozygous will not beaffected

If both parents have thecondition, all offspringwill be affected

Albinism

QUESTION: If bothparents are carriers canthey have albinochildren?


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Sex-linked traits

Controlled by loci on sex chromosomes

X or Y

Hemophilia


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Males can be HEMIZYGOUS for a trait, such as hemophilia

- they can have only one copy of the allele AND express the

trait because the X and Y chromosomes are not truly

homologous

(These are

chromosomes that have

replicated during cell

division.)


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Mutation

Alice

Alexandria and Tzar Nicholas


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Orange = 75% are male

Tortoiseshell and calico =always female; if male are

sterile


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Non-Mendelian inheritance

Pleiotropy one gene influences more than one trait

Polygenic inheritance (NON-monogenic)

multiple genes for one trait includes most traits of the skeleton

Genotype = phenotype

strong environmental effects

Also: Mitochondrial inheritance


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Pleiotropy

A single geneinfluences more than

one phenotypic

expression

Phenylketonuria


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Environmental influence Many polygenic traits are influenced by the environment

Most Mendelian traits will not be influenced by theenvironment

Can human culture affect

gene expression?


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Mitochondrial inheritance

Mitochondria in cytoplasm of cells

mtDNA directs the conversion of energy

within the cell

Mother to offspring transmission only

Mitochondrial Eve


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Modern Synthesis

A synthesis of Mendels and

Darwin/Wallaces views was not

achieved until the mid-1930s

Modern Synthesisevolution defined as

a two stage process

1. The production and redistribution of variation

2. Natural selection acting on this variation andaffecting reproductive success

4. mendelian inheritance

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