Brainteaser

Which way is the bus below traveling...toward the left or the right?

Sex linked traits and Pedigrees

_X_-Linked Traits• Sex-linked traits are produced by genes only on the X

chromosome. • They can be Dominant or Recessive.

• A = dominant a = recessive

• What would be the genotypes of a male and female that have a Sex-linked Dominant trait and do not express the trait?

• Expresses Trait: Male - XA Y Female - XA XA or XA Xa

• No Expression: Male - Xa Y Female - Xa Xa

• What would be the genotypes of a male and female that have a Sex-linked Recessive trait and do not express the trait?

• Expresses Trait: Male - Xa Y Female - Xa Xa

• No Expression: Male - XA Y Female - XA XA or XA Xa (Carrier)

• Most Sex-linked traits are Recessive!

Red-green Color blindness

• X chromosome has a locus for colour vision with two alleles:– XN = Normal colour vision– Xn = Red-green colour blindness

• Y chromosome does not have a colour vision locus.

• If a male receives the Xn allele he will have impaired colour vision, whereas a female with XNXn will not.

Red-green color blindnessParental

Phenotypes Carrier Female x Normal MaleGenotypes XNXn XNYGametes

Offspring 1Genotypes

Phenotypes Normal Female : Carrier Female : Normal Male : Colour blind Male 1 : 1 : 1 : 1

XN Xn XN Y

Female Gametes

XN Xn

Male Gametes

XN XN XN XN Xn

Y XN Y Xn Y

Colorblindness Tests

Test Name: Ishihara Test

Colorblind sees:

yellow square

Normal color:

yellow square &

faint brown circle

Colorblind sees:

the number 17

Normal Color sees:

the number 15

X-linked Disorders

Examples:

colorblindness, baldness, hemophilia

*Occurs more often in MALES than females.

What is a Pedigree?

def: a chart of the genetic history of a family over several generations.

*It shows the patterns of inheritance in a family.

*It’s used by a scientist or genetic counselor to study and predict patterns of inheritance.

How to Construct a Pedigree?

• A Pedigree is a chart showing the pattern of inheritance for a trait. (Family tree)

• Symbols and Rules:

• Male = Female =

• Affected = Unaffected = Carrier =

• Link parents together with a line and then make a vertical line to connect to offspring.

Constructing and Reading a pedigree

a clear “CIRCLE” stands for a HEALTHY female

a clear “SQUARE” stands for a HEALTHY male

*a circle w/a line through, means that family

member has deceased or died.

Constructing and Reading a pedigree

a darkened in “CIRCLE” stands for a “AFFECTED” female

a darkened “SQUARE” stands for a “AFFECTED” male

*Affected means the person has the disorder.

a HALF-darkened in “CIRCLE” stands for a “CARRIER” female.

a HALF-darkened “SQUARE” stands for a “CARRIER” male.

*CARRIER means the person has an allele for the disorder.

Constructing and Reading a pedigree

*a vertical line means the married couple had” offspring, kids…

a horizontal line between a male and female means

the persons are married, “husband and wife”.

Constructing and Reading a pedigree

*Roman Numerals (I, II, III) refers to the generations.

*Arabic numbers refers to individuals. (1, 2, 3, 4, 5, …)

321 4 5 6

1

1

2

2

3 4 5

I.

III.

II.

Reading the pedigree…

How many generations are there?How many children did II-1 have? II-7?

How are III-5 and III-2 related?Who is III-2 in reference to I-2?

321 4 5

61

1

2

2

3 4 5

I.

III.

II.7

Types of Pedigrees

Is the pedigree autosomal or X-linked. Pedigrees can be:

a.) autosomal

*There is a 50/50 ratio between men and women of affected individuals.

b.) X- linked

*Most of the males in the pedigree are affected.

Step One:

Autosomal Dominant Pedigree

• Draw a Pedigree showing a cross between Heterozygous parents that have 2 boys and 2 girls. (Show all possibilities)

Genotypes of Affected and Unaffected:

• AA and Aa = Affected aa = Unaffected

Aa Aa

Aa Aa AAaa

Autosomal Recessive Pedigree

• Draw a Pedigree showing a cross between Heterozygous parents that have 2 boys and 2 girls. (Show all possibilities)

Genotypes of Affected and Unaffected:

• AA=Unaffected Aa=Carrier, Unaffected aa=Affected

Aa Aa

Aa Aa AAaa

X-Linked Recessive Pedigree• Draw a Pedigree showing a cross between a

Red eyed Male fruit fly and a Carrier Female fruit fly which have 2 males and 2 females. (Show all possibilities) Red is dominant to white.

• Genotypes of Parents:• Male = XR Y Female = XR Xr

XRY XRXr

XRY XrY XRXR XRXr

Is it Autosomal or X-Linked…

Autosomal, equal numbers of males and females are affected.

3

21 4

5 61

1

2

2 3

4

3

I.

III.

II.

X-Linked Pedigree

X-Linked Recessive Disorders

*Females w/one gene are carriers.

*Females w/two genes are affected.

In X-linked recessive disorders, whenthe mother is a carrier, each malechild has a 50% chance of having the disorder.

If the father has the disorder and isable to have children, boys won't beaffected, because they receive only a Ychromosome from him.

Girls receive his X chromosome andwill be carriers.

Is it Dominant or Recessive…

Dominant, only one parent has the disorder.

3

21 4

5 61

1

2

2 3

4

3

I.

III.

II.

Is it Dominant or Recessive…

Recessive, neither parent has the disorder. Both are heterozygous.

3

21 4

5 61

1

2

2 3

4

3

I.

III.

II.

In Summary

Function: to predict the chances of offspring having a genetic disorder.

Steps:1.) Determine if the disorder is autosomal or X-Linked2.) Determine if the disorder is dominant or recessive.

Hemophilia is a recessive disorder (Xh) that is sex-linked and occurs on the X gene. Which offspring

will likely develop hemophilia?

A. offspring 2

B. offspring 4

C. offspring 1 and 2

D. offspring 3 and 4