Medical Genetics

Medical Genetics

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Mitosis

• Produces daughter cells with 46 chromosomes• Used in growth and repair


Mitosis

• DNA is duplicated• Doubled

chromosomes form from duplicated DNA

• Each cms has 2 identical chromatids


Chromatid

Chromatid

Chromosomes line up in a single row.

Mitosis Metaphase


Chromosomes separate

Each chromatid becomes a single chromosome


Meiosis

• Reduce the chromosome number to half that of body cells

• Produce gametes– Egg– Sperm


Chromosomes line up in a double row.

Meiosis Metaphase


Chromosomes separate

Each each daughter cell gets doubled chromosomes


Doubled Chromosomes Separate in Second Meiotic Division


Mitosis Metaphase

Meiosis Metaphase


Cms 1 Cms 1

Cms 2 Cms 2

Double Filed Chromosomes

• Daughter cells receive ONE of each cms pair

• Daughter cells receive ONE allele for most traits

• New combinations of alleles possible


Gene

• A unit of heredity that controls the development of one trait

• Made of DNA


Allele

• Member of a paired gene– One allele comes from each parent

• Represented by a single letter


Dwarfism = D

Normal height = d

DD = Dwarfism

Dd = Dwarfism

dd = Normal height

Examples of Alleles


Dwarf Band

http://www.quigsmss.com/kingzhomepage.htm

Dominant & Recessive Alleles

• Dominant alleles are expressed• Recessive alleles are not expressed in the

presence of a dominant allele– Recessive alleles are only expressed if both

recessive alleles are present


Homozygous

• Both alleles alike• AA or aa


Heterozygous

• Alleles are different• Aa


Genotype

• Genetic make up• Represented by alleles• DD & Dd are genotypes for dwarfism


Phenotype

• A trait• Genotype determines the phenotype• Dwarfism is a phenotype


Codominant

• Two different alleles are both dominant• A = allele for type A blood• B = allele for type B blood• AB = results in type AB blood


Karyotype

• Picture of chromosomes from an individual


Homologous Chromosomes

• Chromosomes of the same pair• Karyotypes are usually arranged with

homologous chromosomes paired together


Mutation

• Change in a gene or chromosome• Causes an abnormal trait


MutagenAgent that causes mutations


Cigarette smoke

Pesticides

X-rays

Ulatraviolet light

Nuclear radiation

Homologous chromosomes line up in a double file in metaphase I of meiosis


Homologous Pairs Separate


Four Gametes With Single Chromosomes


Fertilization


Nondisjunction


Trisomy


Sex Chromosomes


Sex Chromosomes

• Male have Xy– Male gametes have either X or y

• Females have XX– Female gametes have X


AutosomesChromosomes 1-22


X-Linked Traits

• Alleles are on the X chromosome• Females have two alleles• Males have one allele

– Only one X chromosome


Normal Male


Normal Female


Trisomy 21Down Syndrome


Down Syndrome

• Large tongue• Flat face• Slanted eyes• Single crease across

palm• Mental retardation

– Some are not


Maternal Age & Down Syndrome


Trisomy 18Edward Syndrome


Edward Syndrome

• Heart defects• Displaced liver• Low-set ears• Abnormal hands• Severe retardation• 98% abort• Lifespan < 1 year


Trisomy 13Patau Syndrome


Patau Syndrome

• Cleft lip and palate• Extra fingers & toes

– polydactylism

• Defects– Heart– Brain– Kidneys

• Most abort• Live span < 1 month


Klinefelter Syndrome


Klinefelter Syndrome

• Breast development• Small testes• Sterile• Low intelligence

– Not retarded


Klinefelter Website

http://klinefeltersyndrome.org/stefan.htm

http://klinefeltersyndrome.org/stefan.htm

Turner Syndrome


Turner Syndrome

• Short• Not go through

pruberty• Produce little estrogen• Sterile• Extra skin on neck


Fetal Testing


Sickle Cell Anemia

• RBCs sickle shaped• Anemia• Pain• Stroke• Leg ulcers• Jaundice• Gall stones• Spleen, kidneys & lungs


Sickle Cell Anemia

• Recessive allele, s codes for hemoglobin S – Long rod-like molecules– Stretches RBC into sickle shape

• Homozygous recessive, ss have sickle cell anemia

• Heterozygous, Ss are carriers


Hemophilia

Blood clotting impairedRecessive allele, h

carried on X cmsX-linked recessive traitMore common in males


Albinism

• Lack of pigment– Skin– Hair– Eyes


Amino Acids Melanin PigmentEnzyme

A a

AA = Normal pigmentation

Aa = Normal pigmentation

aa = Albino


PKU Disease

• Phenylalanine excess• Mental retardation if

untreated


Molly’s Story

http://www.peds.umn.edu/pku/Stories.html

http://www.peds.umn.edu/pku/Stories.html

Phenylalanine TyrosineEnzyme

P p

PP = Normal

Pp = Normal

pp = PKU


A man & woman are both carriers (heterozygous) for albinism. What is the chance their children will inherit albinism?


AA = Normal pigmentation

Aa = Normal pigmentation (carrier)

aa = Abino

Man = Aa Woman = Aa

A

a a

A


A

a

a

A AA

Aa

Aa

aa


AA

Aa

Aa

aa

Genotypes1 AA, 2Aa, 1aa

Phenotypes

3 Normal

1 Albino

Probability

25% for albinism


A man & woman are both carriers (heterozygous) for PKU disease. What is the chance their children will inherit PKU disease?


p

p

P PP

Pp

Pp

pp

P

PP = Normal

Pp = Normal (carrier)

pp = PKU disease


PP

Pp

Pp

pp

Genotypes1 PP, 2Pp, 1pp

Phenotypes

3 Normal

1 PKU disease

Probability

25% for PKU disease


A man with sickle cell anemia marries a woman who is a carrier. What is the chance their children will inherit sickle cell anemia?


s

s

s Ss

Ss

ss

ss

S

SS = Normal

Ss = Normal (carrier)

ss = Sickle Cell


Ss

Ss

ss

ss

Genotypes2 Ss, 2ss

Phenotypes

2 Normal (carriers)

2 Sickle cell

Probability

50% for Sickle cell


A man with heterozygous dwarfism marries a woman who has normal height. What is the chance their children will inherit dwarfism? Dwarfism is dominant.


d

d

D Dd

dd

Dd

dd

d

DD = Dwarf

Dd = Dwarf

dd = Normal


Dd

dd

Dd

dd

Genotypes2 Dd, 2dd

Phenotypes

2 Normal

2 Dwarfs

Probability

50% for Dwarfism


X-linked Recessive Traits

• Alleles are on the X chromosome• Inheritance pattern different in males and

females


XH XH = Normal Female

XH Xh = Normal Female (Carrier)

Xh Xh = Hemophilic Female

XHy = Normal Male

Xhy = Hemophiliac Male


A man with hemophilia marries a normal woman who is not a carrier. What is the chance their children will inherit hemophilia? Hemophilia is X-linked recessive.


y

XH

Xh XH Xh

XH

Xh XH = Normal Female



XHy = Normal Male


XH Xh

XHy XHywww.freelivedoctor.com

Genotypes

2 XH Xh, 2XHy

Phenotypes

2 Carrier Females

2 Normal Males

Probability

O% for Hemophilia

y

XH

Xh XH Xh

XH

XH Xh

XHy XHy


A normal man marries a normal woman who is a carrier for hemophilia. What is the chance their children will inherit hemophilia?


y

Xh

XH

XH

Xh XH = Normal Female



XHy = Normal Male


XH XH XH Xh

XHy Xhywww.freelivedoctor.com

Genotypes

XH XH , XH Xh, XHy, XhyPhenotypes

2 Normal Females

1 Normal Males

1 Male Hemophiliac

Probability50% for Male Hemophilic

0% for Female Hemophilic

y

Xh

XH XH XH

XH

XH Xh

XHy Xhy


The End


Medical Genetics

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