Modern GeneticsModern Genetics

Your note outlineYour note outline

1- Morgan and Drosophila1- Morgan and Drosophila 2- Sex determination and chromosomes2- Sex determination and chromosomes 3- Sex linked traits3- Sex linked traits 4- Sex linked human disorders4- Sex linked human disorders 5- Autosomal dominant human disorders5- Autosomal dominant human disorders 6- Autosomal recessive human disorders6- Autosomal recessive human disorders

T.H. Morgan and T.H. Morgan and DrosophilaDrosophila

Fruit flies have Fruit flies have only 4 only 4 chromosomeschromosomes

Easy to storeEasy to store 14 day life 14 day life

cyclecycle

Sex Determination and Sex Determination and ChromosomesChromosomes

Each human cell contains 46 Each human cell contains 46 chromosomes:chromosomes:

22 pairs of autosomes22 pairs of autosomes

1 pair of sex chromosomes1 pair of sex chromosomes

Sex chromosomes are designated Sex chromosomes are designated as: as:

“ “X” and “Y”X” and “Y”

XX= femaleXX= female

XY= maleXY= male

The sex of a human is genetically The sex of a human is genetically determined at fertilization when a determined at fertilization when a sperm cell containing sperm cell containing eithereither the X the X or Y chromosome unites with an or Y chromosome unites with an

egg cell containing the X egg cell containing the X chromosome.chromosome.

Sex DeterminationSex Determination

Female and Male Sex Female and Male Sex ChromosomesChromosomes

X X X

Y

Cc

C

H

s

c

h

S

C

H

s

Who gives the sons the eye color gene? Mom or Who gives the sons the eye color gene? Mom or Dad?Dad?

Sex-Linked TraitsSex-Linked Traits

Morgans work with Morgans work with DrosophilaDrosophila demonstrated that genes for demonstrated that genes for certain traits are located on certain traits are located on the X chromosome and do not the X chromosome and do not appear on the Y chromosome.appear on the Y chromosome.

Genes found on the Genes found on the X X chromosome are said to be chromosome are said to be sex-linked genessex-linked genes

Recessive traits that Recessive traits that are sex-linked occur are sex-linked occur more frequently in more frequently in malesmales than in females. than in females.

For the recessive trait For the recessive trait to show in a female, to show in a female, she must be she must be homozygous homozygous recessiverecessive..

The gene must be The gene must be present on present on bothboth of her of her X chromosomes( X X).X chromosomes( X X).

Human Disorders Human Disorders associated with Sex-Linked associated with Sex-Linked

GenesGenes1.1. HemophiliaHemophilia- disease in which the blood - disease in which the blood

does not clot properly.does not clot properly.

2.2. ColorblindnessColorblindness- inability to see certain - inability to see certain colors, most commonly red and green.colors, most commonly red and green.

* Both of these disorders are * Both of these disorders are more common more common in malesin males than in females because a than in females because a female will not show the disorder as female will not show the disorder as long as she has one normal gene. long as she has one normal gene. Females who are Females who are heterozygousheterozygous for a sex-for a sex-linked trait are said to be linked trait are said to be carrierscarriers for for that trait.that trait.

Color blind punnett squareColor blind punnett square

C = normal color vision

c = color blind

Answer these questions Answer these questions Which child is colorblind? Explain in Which child is colorblind? Explain in

terms of the alleles he or she terms of the alleles he or she inherits. inherits.

Which child is a carrier? Explain in Which child is a carrier? Explain in terms of the alleles he or she terms of the alleles he or she inherits. inherits.

If the If the fatherfather had been red-green had been red-green colorblindcolorblind and and the the mothermother was still was still a a carriercarrier, which children would be , which children would be colorblind? Which children would be colorblind? Which children would be carriers? Can a son be a carrier? carriers? Can a son be a carrier? Create a Punnett square to find out.Create a Punnett square to find out.

hemophiliahemophiliaH = normal

h = hemophilia

What do you think?What do you think?Can a father with hemophilia Can a father with hemophilia

pass the disease on to his son pass the disease on to his son if the mother is homozygous if the mother is homozygous for the normal allele?for the normal allele?

What will the genotype of the What will the genotype of the daughters be?daughters be?

What percentage of the What percentage of the daughters will be carriers of daughters will be carriers of the disease?the disease?

Autosomal Dominant Autosomal Dominant DisordersDisorders Normally Normally two working copiestwo working copies of of

every gene in each every gene in each individualindividual. . In dominant genetic disease, In dominant genetic disease, one one

copycopy of the gene is altered by of the gene is altered by mutationmutation and causes the disease and causes the disease

achondroplasiaachondroplasia (a form of dwarfism), (a form of dwarfism), neurofibromatosis, and neurofibromatosis, and Huntington diseaseHuntington disease..

Try some examplesTry some examples

(3) Joan and John both have (3) Joan and John both have achondroplasia. They are trying to decide achondroplasia. They are trying to decide whether to have children. What is the whether to have children. What is the probability that any fetus they produce probability that any fetus they produce will be homozygous dominant for will be homozygous dominant for achondroplasia and suffer skeletal achondroplasia and suffer skeletal abnormalities causing a spontaneous abnormalities causing a spontaneous abortion? abortion?

25%25% AAAA AaAa

AaAa aaaa

A

A

a

a

(4) Zach's mother has achondroplasia but (4) Zach's mother has achondroplasia but Zach does not. What is the probability of Zach does not. What is the probability of Zach passing achondroplasia to his Zach passing achondroplasia to his children?children?

0% 0% (5) Margaret, who is normally statured is (5) Margaret, who is normally statured is

married to a man who has achondroplasia. married to a man who has achondroplasia. What is the probability that their first child What is the probability that their first child will have achondroplasia? will have achondroplasia?

50%50%

Autosomal DominantAutosomal Dominant

parent who shows the trait will pass parent who shows the trait will pass the mutation on to the mutation on to half half of his/her of his/her children with an children with an equal chanceequal chance for for sons and daughters to be affected. sons and daughters to be affected.

Children who do not have the trait Children who do not have the trait will generally will generally not passnot pass the disease the disease on to their children. on to their children.

observed in each generationobserved in each generation, usually , usually without skipping a generation. without skipping a generation.

Autosomal Recessive Autosomal Recessive DisordersDisorders

both parents must be carriers (i.e., they both parents must be carriers (i.e., they are clinically normal but have one are clinically normal but have one mutation of a particular gene)mutation of a particular gene)

both must pass the mutation to a child in both must pass the mutation to a child in order for that child to be affected. order for that child to be affected.

This inheritance pattern is distinctive in This inheritance pattern is distinctive in that the parents and other relatives of the that the parents and other relatives of the person with the disease appear to be person with the disease appear to be completely normal, while 25% of their completely normal, while 25% of their brothers and sisters will share the same brothers and sisters will share the same disease disease

Autosomal RecessiveAutosomal Recessive

mutation travels unobserved mutation travels unobserved (silently) within the family and is (silently) within the family and is expressed by siblings in a single expressed by siblings in a single generation generation

sickle cell anemiasickle cell anemia, , cystic fibrosiscystic fibrosis, , TayTay-Sachs disease-Sachs disease, and , and phenylketonuriaphenylketonuria..

Gene LinkageGene Linkage Every organism has Every organism has thousandsthousands of genes of genes Genes that are on the Genes that are on the same same

chromosomeschromosomes make up a make up a linkage linkage groupgroup

Humans have Humans have 23 linkage groups23 linkage groups Linked genes Linked genes do notdo not separate separate

independentlyindependently Linked genes will not follow the Linked genes will not follow the

dihybrid ratio of 9:3:3:1dihybrid ratio of 9:3:3:1 What’s going on here?What’s going on here?

Crossing Over Crossing Over (not this kind (not this kind ))

ExchangeExchange of pieces of of pieces of homologous chromosomeshomologous chromosomes

Occurs during Occurs during synapsissynapsis of the of the first meiotic division when first meiotic division when chromotids are in close contactchromotids are in close contact

Distant genesDistant genes are separated are separated more often than closely linked more often than closely linked

Creates new gene linkagesCreates new gene linkages Thus new variations (think Thus new variations (think

genetic diversitygenetic diversity and and evolution!)evolution!)

bbcc bbcc xx BBCC BBCC becomes becomes bbcC bbcC xx BBcC BBcC

Multiple Gene Inheritance Multiple Gene Inheritance (Polygenic)(Polygenic) When When 2 or more2 or more independent genes independent genes

affect affect one characteristicone characteristic Example:Example:

– If the length of an ear of corn was If the length of an ear of corn was controlled by two genes Aa and Bb with controlled by two genes Aa and Bb with A and B being dominant for long, the A and B being dominant for long, the dihybrid cross would yield a range of dihybrid cross would yield a range of phenotypes between pure dominant and phenotypes between pure dominant and pure recessive.pure recessive.

– The ear with the The ear with the most dominant lettersmost dominant letters is the longest (AABB) is the longest (AABB)

– What would the genotype be of the What would the genotype be of the shortest ear?shortest ear?