genetics the study of heredity. genetics genetics is the scientific study of heredity - how traits...

Genetics

The study of heredity

Genetics

Genetics is the scientific study of heredity - how traits are passed from generation to generation.

The characteristics that are inherited are called traits.

Genes Humans have 23

homologous pairs of chromosomes.

On each chromosomes, there are sections called genes that code for traits.

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Genes



Alleles An allele is a distinct form of a gene.

Every person has 2 alleles for a gene 1 from the father and 1 from the mother


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Alleles

Letters of the alphabet are used to represent an allele of interest.

Every person has two copies of an allele, so they will have two letters.

AP

R

T

Y

Alleles

Dominant alleles are symbolized with a capital letter. Dominant alleles will mask a recessive alleles in cases of simple dominance/recessiveness.

Recessive alleles are symbolized with lower case letters.

A

a

Homozygous Alleles

If an organism has two like copies of an allele, it is homozygous (homo = same).

If the two alleles are dominant, the organism is homozygous dominant.

If the two alleles are recessive, the organism is homozygous recessive.

AA

aa

Heterozygous Alleles

If an organism has two different copies of an allele, it is heterozygous (hetero = different).

Aa

Genotype and Phenotype

The letters an organism has represent the organism’s genotype - what alleles the organism has.

As a result of the alleles present, a trait is expressed. The phenotype is the expressed trait.


Example: In a plant species, there are two alleles for flower color: R and r.

R is dominant, and codes for red flowers

r is recessive and codes for white flowers




The genotype is the combination of alleles: either RR, Rr, or rr.

The phenotype is what is expressed: either red or white flowers.




RR - Homozygous dominant Red flowers

Rr - Heterozygous dominant Red flowers

rr - Homozygous recessive White flowers

In cases of simple dominance, an organism must have two copies of the recessive alleles to express the recessive trait.

Purebreds and Hybrids

Purebred - an organism that receives the same genetic traits from both of its parents

Hybrid - an organism that receives different forms of a genetic trait (different alleles) from each parent

Mendel’s Laws

Contributions of Gregor Mendel

Law of Dominance

The dominant alleles is expressed and may mask a recessive allele. The recessive form of a trait is only shown in a homozygous recessive organism.

Ex. R is allele for round, r is allele for square. RR - round Rr - round rr - square

Law of Segregation

Gene pairs separate when gametes are formed.

Parent:Dd

D d

Gametes

Parent:dd

d d

Gametes

Law of Independent Assortment

Genes segregate randomly and independently. This means that if there are 2 or more traits, every combination of those traits is possible.

AabbCc

AbCAbcabCabc

Probability and Punnett Squares

Predicting the genotypes and phenotypes of offspring

Probability

Probability - the likelihood that a particular event will occur (what are the odds?)

What is the probability that a single coin flip comes up heads? 50% or 1/2

Probability

True or False? The past outcomes of coin flips greatly affects the outcomes of future coin flips.

False. There’s still a 50% chance of heads and

50% chance of tails!

Probability

The way in which alleles separate is random, like a coin flip. (Mendel’s Law of Segregation)

From a mother who is heterozygous for an allele, there is a 50% chance she passes on the dominant allele and a 50% chance she passes on the recessive allele.

Punnett Squares

Punnett squares show probabilities for genotypes and phenotypes of offspring of two parent organisms.

Example: In Mendel’s pea plants, the plants had

either purple (P) or white (p) flowers. QuickTime™ and a


Punnett Squares

Step 1. Make the grid. If there is 1 trait, it is a 2x2 grid. If there are 2 traits, it is a 4x4 grid.

Because we are only looking at 1 trait (flower color), a 2x2 grid is needed.

Punnett Squares

Step 2: Determine the parents’ genotypes and possible gametes.

Example: a heterozygous pea plant and a homozygous dominant pea plant.

Pp

PP

P

P

P p

Punnett Squares

Step 3: Fill in the squares by combining what is on top of the column and to the left of the row.

Pp

PP

P

P

P p

PP

PP

Pp

Pp

Punnett Squares

Step 4: Use the Punnett square to determine probabilities and ratios.

Pp

PP

P

P

P p

PP

PP

Pp

Pp

Punnett Squares What is the probability of

an offspring plant having purple flowers? 100%

What is the probability of an offpsring plant being heterozygous? 2/4 = 1/2 = 50%

PP

PP

Pp

Pp

Punnett Squares If there are 2 traits, the Punnett square will

be a 4x4 grid.

Example: Cross a pea plant that is heterozygous for both flower color and seed shape with a plant that has white flowers and is heterozygous for seed shape

P - purple; p - white

R - round, r - wrinkled

Punnett Squares Cross a pea plant that is heterozygous for

both flower color and seed shape with a plant that has white flowers and is heterozygous for seed shape

PpRr ppRr

PR, Pr, pR, pr pR, pr, pR, pr

Punnett Squares

PpRr

ppRr

PR

Pr

pR

pr

pR pR pr pr

Punnett Squares

PpRr

ppRr

PpRR

PpRr

ppRR

ppRr

PR

Pr

pR

pr

pR pR pr pr

Punnett Squares

PpRr

ppRr

PpRR PpRR PpRr PpRr

PpRr PpRr Pprr Pprr

ppRR ppRR ppRr ppRr

ppRr ppRr pprr pprr

PR

Pr

pR

pr

pR pR pr pr

Punnett Squares

What is the probability of an offspring having white flowers and wrinkled seeds?

PpRR PpRR PpRr PpRr

PpRr PpRr Pprr Pprr

ppRR ppRR ppRr ppRr

ppRr ppRr pprr pprr

PR

Pr

pR

pr

pR pR pr pr

2 / 16 = 1 / 8

or

12.5%

Punnett Squares

What is the probability of an offspring having purple flowers and round seeds?

PpRR PpRR PpRr PpRr

PpRr PpRr Pprr Pprr

ppRR ppRR ppRr ppRr

ppRr ppRr pprr pprr

PR

Pr

pR

pr

pR pR pr pr

6 / 16 = 3 / 8

or

37.5%

Punnett Squares

Write the probable genotypic ratio.

2 PpRR : 4 PpRr : 2 Pprr : 2 ppRR : 4 ppRr : 2 pprr 1 PpRR : 2 PpRr : 1 Pprr : 1 ppRR : 2 ppRr : 1 pprr

PpRR PpRR PpRr PpRr

PpRr PpRr Pprr Pprr

ppRR ppRR ppRr ppRr

ppRr ppRr pprr pprr

Intermediate Inheritance

Beyond Simple Dominance

Intermediate Inheritance

There are 3 types of intermediate inheritance, genetic patterns that don’t follow the simple dominant-recessive rules.

Incomplete dominance Codominance Multiple alleles

Incomplete Dominance

Incomplete dominance - neither allele is completely dominant over the other

The heterozygous form is a “blended” form of the two alleles.


Example: In snapdragon flowers, there is an allele that codes for red (r), and allele that codes for white (w).

rr - red ww - white rw - pink




Ex. Cross a red and a pink snapdragon.

rr rw

rr rw

r

r

r w

Incomplete Dominance Sometimes two like capital letters are used, but one gets a prime sign (‘).

Ex: Human hair Curly hair HH Straight hair H’H’ Wavy hair HH’

Codominance

Codominance - both alleles are dominant and get expressed equally

In the heterozygous has some of each phenotype, but they are not blended.

Codominance

Example - in a type of cattle, red hair (R) and white hair (W) are codominant.

RR - red WW - white RW - roan

Some red, some white, but not pink!



Codominance Ex. Cross a red parent and a white

parent.

RW RW

RW RW

W

W

R R

Multiple Alleles

Multiple alleles - there are more than 2 alleles for a trait.

Ex. Fur color - gray, black, striped

Ex. Human blood typesQuickTime™ and a


Sex Linkage

Sex-linked, sex-limited,

and sex-influenced traits

Human Chromosomes

Humans have 23 homologous pairs of chromosomes, for a total of 46.

22 pairs are called autosomes , which are all of the non-sex chromosomes

The 23rd pair is the sex chromosomes - X and Y.

Sex Chromosomes

X and Y

Females - XXAll eggs have an X

Males - XYSperm have either an X or Y



Sex-Linked Traits Traits controlled by genes on the sex

chromosomes are sex-linked traits.

Examples of sex-linked traits: hemophilia, color blindness, male pattern baldness

Most are “attached” to the X chromosome. Therefore, females have 2 copies of

these alleles and males only have one

Example - Hemophilia Hemophilia - a blood clotting disorder

Hemophilia is X-linked.

XH = normal Xh = hemophilia

Y is still just a Y QuickTime™ and aTIFF (Uncompressed) decompressor


Example - Hemophilia Females could be:

XHXH - don’t have hemophilia, not a carrier

XHXh - don’t have hemophilia, is a carrier

XhXh - have hemophilia



Example - Hemophilia

Males can be:

XHY - does not have hemophilia XhY - has hemophilia

Males cannot be carriers - they either have it or they don’t!



Example - Hemophilia Draw a Punnett square for cross between a carrier

female and an unaffected male. Female: XHXh Male: XHY

XHXH XHY

XHXh XhY

XH

XH

Y

Xh

Example - Hemophilia What is the percent chance that a child of theirs

will have the disorder? 25%

XHXH XHY

XHXh XhY

XH

XH

Y

Xh

Example - Hemophilia What is the percent chance that a a son would

have the disorder? 50%

XHXH XHY

XHXh XhY

XH

XH

Y

Xh

Example - Hemophilia What is the percent chance that a daughter

would be a carrier? 50%

XHXH XHY

XHXh XhY

XH

XH

Y

Xh

Example - Colorblindness

Color blindness is also X-linked.

X = normal Xc = colorblind

Example - Colorblindness

Cross a colorblind male and a carrier female.

XXc XY

XcXc XcY

Xc

X

Y

Xc

Sex-limited traits

Sex-limited traits are only expressed in the presence of sex hormones, or are only observed in one sex or the other.

Ex. Beard growth



Sex-influenced traits

Sex-influenced traits are expressed in both sexes, but they are expressed differently.

Ex. Baldness is dominant in men, recessive in women



Pedigrees

Pedigrees

Males Females

Affected - shaded Unaffected - not shaded Carrier - half shaded

Pedigrees

A pedigree is a diagram showing family history and tracing a genetic trait.



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