Topic 5: Genetics • Heredity is the study of

inheritance which is controlled by our genes and genes are found on chromosomes

• Each person has 2 genes (alleles) for each trait – one from each parent

What patterns do you notice in the table?

What is a gene pool?

All the genes in a given population

Terminology you need to know

Terminology you need to know• Allele- alternate forms of a gene controlling a

characteristic. Alleles are found in the same position (loci) of homologous chromosomes

• Dominant allele: masks the effect of the recessive allele. Can be expressed in heterozygous form. Represented by an upper case letter:

• Recessive allele: masked by the effect of the dominant allele. Can only be expressed in homozygous form. Represented by a lower case letter:

We inherit one allele from each parentEach trait is controlled by at least 2 alleles

Ex. The gene is tongue rolling, the alleles can be for tongue roller or non tongue roller

T

t

T

t

Do you have hitchhikers thumb?

If you do, you have the two recessive alleles (tt), one from mom, the other from dad

If you don’t, you have at least one dominant allele (Tt or TT), the dominant allele could be from mom or dad

Page 13 of the data booklet

Do you have a hitchhiker’s thumb?

More terminology• Genotype – alleles that make up a trait (must be at least 2 letters)

• E.g. Genotype for tongue rolling could be Tt or TT, non-tongue rollers have a tt genotype

• Phenotype – observable traits of an organism (physical appearance) dependent on its genotype, but can be affected by our environment

» ex: freckles or no freckles

• Homozygous dominant – genotype with 2 dominant alleles

• Homozygous recessive - genotype with 2 recessive alleles

• Heterozygous – genotype with 1 dominant and 1 recessive allele• This individual is a carrier for the recessive allele

tt

Tt

TT

Gregor MendelFather of Modern Genetics

and Austrian MonkStudied and breed pea plants

1. They can self-fertilize and/or cross-fertilize breeding easy to control

2. They have several traits that are expressed in two distinct forms–

– different coloured seeds (yellow vs. green),

– size of plants (tall vs. short), position of flowers etc.

Why garden peas?

• What is the phenotype of the following:» RR? » Yy? » gg?

Pea traits that Mendel studied

Round seed (homozygous dominant)

Yellow seed (heterozygous)

Yellow pod (homozygous recessive)

Summary of Mendel’s Cross

• Mendel initially crossed a pure tall and pure short plant together and noticed that all offspring are tall

• He then took 2 of the offspring and breed them and noticed that ~75% of the offspring were tall and ~25% were short

• He noticed the same results every time he performed the experiments

• We can now use Punnett Squares to predict the outcomes of different mating's

Predicting the outcome of a cross?

Steps to a Punnett Square:1.Write down a legend of the letters that you will use to indicate dominant and recessive alleles

1.Write down the genotypes for the parents (write down the given and leave blanks for unknown)2.Show the different gametes that each parent will produce (circle them!!)

3.Use a Punnett Square to show the results4.Answer the question!

T = Tall

t = short

TT x tt

T T t tx

• Before we use a Punnett square to predict outcomes of Mendel’s cross, we must define the following:

Parents referred to as P1 (P)

First generation referred to as F1 (F=Filial)

Second generation referred to as F2

Tt Tt

Tt Tt

Cross 1: Pure tall x Pure short(make sure you write all of this down)

What percentage of offspring are tall?

What is their genotype?

1. Legend:

2. Genotype of parents: 3. Gametes:

T = Tall

t = short

TT x tt

T T t t t

t

TT

Offspring

Gametes

100% Tall with a genotype of Tt

TT Tt

Tt tt

What is the probability of tall offspring? What is their genotype?

Tall offspring: 0.75 or 75% Genotype: TT and Tt

What happens when offspring from the first cross (F1)

generation are bred (Tt x Tt)?tT

t

T

The dominant trait usually shows up more often as it is

produced by 2 possible genotypes, Tt or TT

1. Legend:

2. Genotype of parents: 3. Gametes:

T = Tall

t = short

Tt x Tt

T t T t

A Punnett square links

together what we learned

about mitosis, meiosis, haploid

and diploid.

Label the picture to the right to see

the connection

Tt tt

Tt tt

1. Legend:

2. Genotype of parents: 3. Gametes:

T = Tall

t = short

Tt x tt

T t t t t

t

tT

The probability of having a short offspring is 50%

What is the probability of a short offspring, from a cross between a heterozygous tall plant and a homozygous recessive short plant?

Another example…Photo Sensitive Sneezing (sneezing when

suddenly exposed to bright light) is a dominant trait. A photo sensitive man

(heterozygous) marries a normal woman.

What is the probability of a normal girl? How about a normal boy?

1. Legend: P = Photo sensitive (dominant)p = normal (recessive)

p p

P

p

Pp Pp

pp pp

Probability of normal: 50%Probability of a girl: 50%

Probability of a normal girl: 0.5 x 0.5 = 0.25 = 25%

2. Genotype of parents: Pp x pp 3. Gametes: P, p and p, p

Pedigree Charts

A pedigree chart shows how traits are passed on in a family (similar to a family tree)

 This info is in your data

booklet on page 13

•The circle represents a __________•The square represents a __________•Shaded shapes represent family members who express the trait or are affected (for example, dimples).• In this example, the ________has dimples.

• A horizontal line is used to connect __________. (mating)

• Vertical lines connect parents and ___________.

• The oldest children are placed on the ________ and the youngest on the

___________. In this example, there are three children — two females and a male. The ________ child has no dimples.

Pedigrees

female.

male.two parents

right

left

children

male

parents

children

female

Your turn: Assume that the trait for straight hair is recessive. Write a legend and then determine the genotype and phenotype of all individuals

Is it possible that for a recessive trait, 2 affected individuals

have a child without the trait?

So, if the straight hair trait is not inherited though a recessive allele, how is it passed on?

Legend: A = curly haira = straight hair

aa, straight hair phenotype

aa, straight hair phenotype

Aa, curly hair phenotype

No, where does the dominant allele come from?

The allele for straight hair is dominant rather than recessive!

Your turn: Assume that the trait for brown eyes is dominant. Write a legend and then determine the genotype and phenotype of all individuals

Is it possible that for a dominant trait, 2 affected individuals

have a child without the trait?

How can you explain this?

Legend: A = Brown eyes a = not brown eyes

A_, brown eyes phenotype

A_, brown eyes phenotype

aa, not brown eyes phenotype The parents are both carriers

(heterozygous, Aa). We can go back and fill in the genotypes of the

parents

Yes

II

I

III

1 2

1 2 3 4 5

321

76

4 5 6

1. Label the generations and each member of the generations.

2. How many children did I-1 and I-2 have? _________________4 children

3. What was their sex? ______________________________Female, female, male, female

4. Which one is the oldest child? _______II-1

5. Assuming that having dimples is the recessive trait and no dimples is the dominant trait. Write the genotypes for all the individuals on the pedigree.

Dd dd

dd Dd Dd dd dd Dd dd

dd dd dd Dd dd dd

Legend: D = No dimplesd = dimples

6. What is the chance that the next child of individuals II-1 and II-2 will have dimples?

d Dd

Ddd

D ddd

ddThe chance that their next child will

have dimples is 50%.

Legend: D = No dimplesd = dimples

Genotype of parents: Dd x dd Gametes: D, d and d, d

7. What is the chance that the next child of II-4 and II-5’s will be a boy with dimples?

d dd

ddd

d ddd

dd

Legend: D = No dimplesd = dimples

Genotype of parents: dd x dd Gametes: d, d and d, d

Probability of dimples: 100% (1.0)Probability of a boy: 50% (0.5)

Probability of a boy with dimples: 1.0 x 0.5 = 0.5 = 50%

The genotype of individual II-4 is _____________

A person represented on the pedigree chart who has a homozygous genotype is ___________

What are the chances that individual II-3 and II-4 have another child who will have PKU? _____________

Can 2 unaffected individuals have an

affected child? How?

Do all individuals of affected parents also have to be affected?

Why?

Pp

II-3 or III-2 and III-3

50%

Yes, they only have recessive alleles

Yes, if they are both carriers

Test Cross• Do you have mid-digit hair?

– If you do not, you have the homozygous recessive genotype (hh)

– If you do, you have at least one dominant gene (H_)

• But how do you know if you are Hh or HH?!?

• A test cross is used to determine the genotype (HH or Hh) of an individual who expresses the dominant characteristic

What exactly is a test cross?• To perform a test cross, we will cross the unknown plant (TT or Tt)

with a homozygous recessive (tt) plant and examine the offspring produced

Testcross with monohybrids

Test Cross #1

Phenotypic RatioAll Tall

Genotypic RatioAll Tt

Tt Tt

Tt Tt

No shortoffspring

Unknown Tall plant (TT or Tt) x short plant (tt)

Unknown is likely TT to produce all tall offspring

t

?

t

?

Test Cross #2

Tt Tt

tt tt

Unknown Tall plant (TT or Tt) x short plant (tt)

Unknown is likely Tt to produce both short AND tall

offspring

Phenotypic Ratio2 Tall : 2 short

Genotypic Ratio2 Tt : 2 tt

Some shortoffspring

t

?

t

?

Gender and Inheritance• While working with fruit flies

(Drosophila melanogaster), Thomas Hunt Morgan, an American Geneticist noticed that there were distinct gender differences in a few things:– Size– Eye colour

female male

Sex-linked inheritance• Traits coded by genes located on the

sex chromosomes are known as sex linked traits

• Male sex chromosomes: XY• Female sex chromosomes: XX

• Sex-linked traits are almost always on the X chromosome

• The Y chromosome is used for sex determination

• Since males only have 1 X chromosome, they cannot be heterozygous or carriers for sex linked traits

Traits controlled by genes carried

on the Y chromosomeonly affect males and are

known as Y-linked

Eye Color In Drosophila• To support the idea that eye color is sex

linked, let’s assume the genes controlling eye colour in fruit flies be on the X chromosome

Let R = red eyes r = white eyes• The Y chromosome does not carry gene

for eye color• Write all the possible genotypes and

phenotypes for eye colour

Ex. XRXR

Mostlymales

Mostlyfemales

Sex-linked alleles are symbolized by superscripts attached to the X or the Y

chromosome

Females Males

XRXR (red)

XRXr (red)

XrXr (white)

XRY (red)

XrY (white)

First Cross

All offspring have red eyes, inherited from mom

Xr XR Xr

XRYY

XRLegend: R = red r = white

Parents:XRXR x XrY

GametesXR, XR and Xr, Y

Homozygous dominant red eyed female and a white eyed male

XR

XR Xr

XRY

XR XR XR

XRYY

XR Xr

XR Xr

XrY

Second Generation (F1 Cross)

Legend: R = red r = white

Parents:XRXr x XRY

GametesXR, Xr and XR, Y

All female offspring have red eyes, one male offspring has red eyes, the other white eyes

Heterozygous red eyed female and a red eyed male

• First cross: No white eyed female offspring are possible since the mom passed on the XR allele to all her children

• Second cross: No white eyed female offspring are possible since daughters inherit their X chromosomes from their fathers, whom all had the XR allele

Conclusions

Cross #1 Cross #2

Xr XR Xr

XRYY

XR Xr

Xr Xr

XrY

Legend: R = red r = white

Parents:XRXr x XrY

GametesXR, Xr and Xr, Y

The chances of having a son with white eyes is 25%.

Practice Problem: Cross a red-eyed female (heterozygous) with a white-eyed male. What are the chances they will have a white-eyed son?

Examples of sex-linked recessive traits in humans

• Sex linked recessive traits (Xr) occur more often in males than females• Why do you think this is true?

• Males only have one X chromosome so they cannot be carriers, they either have the trait or not

• Examples of sex-linked recessive inheritance are: • Red-green color blindness• Hemophilia - Trouble clotting blood caused

by the lack of a blood protein, called Factor VIII, found on the X Chromosome

Can you identify these 2 numbers?

Xc XC Xc

XCYY

XC Xc

Xc Xc

XcY

Legend: C = normal c = colourblindParents:

XCXc x XcY

Gametes

XC, Xc and Xc, Y

What is the probability that their first child will be a normal son?

Practice Problem: Colour-blindness is a sex-linked recessive disorder. A man with color-blindness is married to a woman who is a carrier of the disorder.

The chances of having a colourblind son is

25%.

Can boys ever inherit an X linked trait from their

dad? Why or why not?

Can a colourblind mother have a normal vision son?

The following is a pedigree for a family in which colour-blindness has been observed.

What is the genotype and phenotype of individual’s I-1 and I-2? Write them on the pedigree

What is the probability that individual’s I-1 and I-2 have a daughter that is colour blind?

Colourblind female

XcXcNormal male

XCY

Do a Punnett Square to answer this.

0%, since the father has normal vision

No. Boys inherit the Y chromosome from dad

No. Since she only has affected alleles to pass on

(Xc) to all her children

• Sometimes in science we are given a pedigree but we don’t know if the trait is inherited through an autosome or on a sex chromosome. To predict the probability (chances) of inheriting a trait we use a few simple tools to first determine the mode of inheritance

Putting it all together - Modes of Inheritance

Is brachydactyly (short fingers and toes) a dominant or

recessive trait? Is the gene found on an X chromosome?

We have now studies 5 different modes of inheritance.

Mode of Inheritance Affected Genotype(s) Y-linked

Autosomal recessive

Autosomal dominant

X-linked recessive

X-linked dominant

Use A and a as the alleles

Only males are affected

Traits are inherited on autosomes

(chromosomes 1-22)

Traits are inherited on the X

chromosome only

XY (affected gene found on the Y

chromosome)

Aa (Aa are carriers)

AA and Aa

XaXa and XaY

XAXA, XAXa and XAY

Steps in determining the mode of inheritance

Only boys are affected and an affected male must have

all affected sons and an affected father

If NOT, then we move on

To determine how a trait is inherited, follow these steps:

Step 1: Is the trait Y-linked?

Autosomal

X- Linked (XD)

If Yes

Affected males have an affected mom and affected daughters

If a trait is not X-linked dominant (XD), then we can say it is autosomal

Dominant

Affected individuals have affected parents

Step 2: Is the trait dominant or recessive?

Autosomal

X- Linked

If Yes

More boys affected. Affected girls have an affected father and an

affected sons

If a trait is not X-linked recessive (XR), then we can

say it is autosomal

Recessive

Affected individual has unaffected parents

Click to watch a tutorial

A famous pedigree… Queen Victoria was a

carrier for hemophilia (an X-linked recessive

disorder)

Check to see if the rules work!

1. Y

XD

AD

XR

AR

Legend:

A =

a =

Y: only males affected, not true (I-1)

Characteristics to go through:

XD or AD: affected individuals have an affected parent (true)

XD: affected males have an affected mom and affected daughters (not true, II-3 is a male, but not all his daughters are affected)

affected

normal

Y

XD

AD

XR

AR

Legend:

A =

a =

Y: only males affected, not true (I-1)

Characteristics to go through:

XD or AD: affected individuals have an affected parent (not true, IV-1 has unaffected parents )

XR: more males affected and affected females have an affected dad and affected sons (not true, more females affected and I-1 is a female, but not all his sons are affected)

Normal

affected

Y

XD

AD

XR

AR

Legend:

A =

a =

Y: only males affected, not true (I-1)

Characteristics to go through:

XD or AD: affected individuals have an affected parent (true)

XD: affected males have an affected mom and affected daughters (true, II-3 is a male, his mom is affected and so are all his daughters)

XA = affected

Xa = normal

3.

Y

XD

AD

XR

AR

Legend:

A =

a =

Y: only males affected, not true (I-1)

Characteristics to go through:

XD or AD: affected individuals have an affected parent (not true, III-1 has unaffected parents )

4.

XR: more males affected and affected females have an affected dad and affected sons (true, 5 males affected, I-1 is a female and her sons are affected and IV-4 has an affected father)

XA = normal

Xa = affected

Tay Sachs Disease

What are the genotypes of all affected individuals?

What is the mode of inheritance?

Tay Sachs is likely to be autosomal recessive. It skip the first generation parents.

It cannot be Y-linked or X-linked recessive because II-1 is an affected girl and does not have an affected father

It cannot be dominant because every affected individual must have at least one affected parent.

Click to watch a tutorial

aa

aa

The trait is likely to be autosomal dominant.

It cannot be Y-linked because II-1 is a girl and affected. It cannot be X- linked dominant because affected males do not have all affected daughters

It cannot be X- linked recessive because affected moms do not have all affected sons

Aa

AaAa

Aa

AaAa Aa

Aa

The trait is likely to be x-linked recessive

It cannot be Y-linked because III-3 is a girl and affected. It cannot be X- linked dominant because affected males do not have all affected daughters

It cannot be autosomal dominant because I-1 and I-2 are not affected but they have affected children

XaY XaY

XaXa

XaY