INTRODUCTION TO LINKED GENES AND SEX LINKAGEH. Biology/ Ms. Kim

Locating Genes on Chromosomes

• Genes– Are located on chromosomes– Code for protein that determines

specific traits.

Figure 15.1

Gene Linkage• Linked genes

– Usually inherited together because located near each other on the SAME chromosome

• Genes closer together on the same chromosome are more often inherited together

• Each chromosome– Has 100’s or 1000’s of genes

• Sex-linked genes exhibit unique patterns of inheritance; genes linked on the X or Y chromosome

What is gene linkage?• Thomas Hunt Morgan (1910) noticed that eye

color in fruit flies and wing size always seemed to be inherited together in offspring.– This seemed to go against Mendel’s Law of

Independent Assortment• Morgan discovered chromosomes are a group

of linked genes• Law of Independent Assortment was still true,

but chromosomes assort independently, not genes.

Gene Linkage• Genes located on the same chromosome will NOT sort

independently• They will be inherited together

– Gene Linkage:• Tendency of 2 alleles (for different traits) to be inherited together• The closer the genes are, the more likely they are to be inherited together

Mendel’s Principle of Independent Assortment?

• Remember: this refers to genes on separate chromosomes

Gene 1

Gene 2

• What would cause genes on the same chromosome to be inherited separately?– Crossing over!

What is gene mapping?

• Location of Gene on a chromosome is it’s LOCUS

Morgan’s Discovery: Sex Linked Traits

• Eye color is linked on X Chromosome• Females carry 2 copies of gene; males

have only 1 copy• If mutant allele is recessive, white eyed

female has the trait on both X’s• White eyed male can not hide the trait

since he has only one X.

How is Sex Determined?

• Male can give either an X or a Y

• Females can only give an X XY

XX XY

MALE

X Y

X

X

FEM

ALE

XXWHAT IS THE PROBABILITY OF HAVING A MALE OR A FEMALE OFFSPRING?

Sex Determination

What is a sex-linked gene?• Sex- Linked Gene:

– Any gene that resides on a sex chromosome– X-linked- gene resides on X chromosome– Y- linked – gene resides on Y chromosome

In humans:- Most sex-linked genes are on the X chromosome- Since, typically the X chromosome is longer, it bears a lot of genes not found on the Y chromosome (Usually on X chromosome called x-linked)

What is a sex-linked gene?

• • The "a" recessive allele will be The "a" recessive allele will not

expressed in his phenotype be expressed in her phenotype

What is a sex-linked gene? (Y-LINKED)

• Genes on the Y chromosome can only be passed to sons. NOT to daughters.

XX XX

XYa XYa

FEMALE

X X

M

ALE

Ya

X

What is a sex-linked gene? (X-LINKED)

Genes on the X chromosome can be passed to either sons or daughters.

• Females can be homozygous or heterozygous.

• Males are hemizygous.

What is the probability of having a son with recessive phenotype? Daughter?

XAY XaY

XAXA XAXa

FEMALE XA Xa

MAL

E

XA

Y

Sex-Linked Traits Practice

• What will the phenotypes be for the following fruit flies?– XRXR

– XrXr

– XRXr

– XRY– XrY

R=Red eyesr=white eyes

Sex-Linked Traits Practice

• White eyes (recessive) are very rare compared to red (dominant) eyes in fruit flies

• Eye color is located on the X chromosome

• Cross a red eyed female (XRXR) with a white eyed male (XrY)

• Now, cross a female and male from the possible offspring produced in earlier cross.

Sex-Linked Traits Question

• If certain recessive disorders, such as color blindness and hemophilia, are sex-linked on the X chromosome, are they more common in males or females? Explain

SEX-LINKED TRAITSWhy are males more often affected by hemophilia?

• Men only have one copy of the X chromosome.

• If a male inherits a mutated gene on the X chromosome, the trait is expressed.

• Females have two copies of the X chromosomes, so the mutated gene may be masked by a normal gene copy.

How is colorblindness a sex-linked trait?

• Red-green colorblindness is caused by light-sensitive cells (cones) in the eyes that aren’t working properly– Individuals who are affected can’t distinguish reds

from greens

Regular Vision Colorblind

What are Sex-linked Disorders?• Usually inherited as sex-linked recessive traits.

– Males need only to inherit one recessive allele to have disorder (XaY)

– Females must inherit two recessive alleles to have disorder ( 1 from mother, 1 from father) (XaXa)

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Pedigree for Colorblindness in a family

What genotype is our 2nd generation female “?”

XBXB XBX?

XBY X?Y

XB

Y

XB X?

She MUST be XBXB

Otherwise, her son would be colorblind!