Human Genetics Phenotype: observed physical and functional traits Genotype: complete set of genes...

Human Genetics

• Phenotype: observed physical and functional traits

• Genotype: complete set of genes and alleles

• Alleles: Different versions of homologous genes ex. B and b

Human genetics

• How are gametes made?• How does chromosome behavior affect inheritance of traits?

• Somatic cells are diploid.• Gametes are haploid, with only one set of chromosomes

egg

polarbody

spermatogonium

primaryspermatocyte

secondaryspermatocyte

oogonium

primaryoocyte

secondaryoocyte

polar bodies(will be degraded)

spermatids

meiosis ll

meiosis l

SPERMATOGENESIS OOGENESISa b

1st law - segregation of alleles

• Cells contain 2 copies (alleles) of each gene

• Alleles separate during gamete formation (meiosis)

• gametes carry only one copy of each gene

Figure 19.2

Possible genotypesand their probabilities

Punnett squares show parental gametesand the genotypes of next generation

•Homozygous: BB and bb•Heterozygous: Bb

Law of Independent Assortment

• During gamete formation, genes for different traits separate independently into gametes

• Why? random alignment of homologues at Meiosis I

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

Figure 9.17

Chromosome behavior accounts for Mendel’s principles


A B

ab

Tetrad

Crossing overA B

a

ba

BA b

Gametes

• Genes on the same chromosome tend to be inherited together = linked genes

• Crossing over produces gametes with recombinant chromosomes


• an offspring’s phenotype is intermediate between the phenotypes of its parents

Incomplete dominance

P GENERATION

F1 GENERATION

F2 GENERATION

RedRR

Gametes R r

Whiterr

PinkRr

R r

R R

r r

1/21/2

1/2

1/21/2

1/2 SpermEggs

PinkRr

PinkrR

Whiterr

RedRR

Figure 9.12A

VARIATIONS ON MENDEL’S PRINCIPLES


• Incomplete dominance in human hypercholesterolemia

Figure 9.12B

GENOTYPES:

HHHomozygous

for ability to makeLDL receptors

HhHeterozygous

hhHomozygous

for inability to makeLDL receptors

PHENOTYPES:

LDL

LDLreceptor

Cell

Normal Mild disease Severe disease


Ex. three alleles for ABO blood type in humans

IA, IB, i

Many genes have more than two alleles in the population


• Quantitative traits

skin color, height, eye color

Polygenic traits - A single trait may be influenced by many genes

Fra

cti

on

of

po

pu

lati

on

Skin pigmentation


• The inheritance of many human traits follows Mendel’s principles and the rules of probability

Genetic traits in humans can be tracked through family pedigrees

Figure 9.8A


• Family pedigrees are used to determine patterns of inheritance and individual genotypes

Figure 9.8B

DdJoshuaLambert

DdAbigailLinnell

D_Abigail

Lambert

Female

DdElizabeth

Eddy

D_JohnEddy

? D_HepzibahDaggett

?

?

ddDdDdDdddDdDd

MaleDeaf

Hearing

ddJonathanLambert


Inherited Genetic Disorders

• Most mutations usually involve recessive alleles

•Phenylketonuria, PKU

•Tay-Sachs disease

•Cystic fibrosis

Figure 9.9A

D D

d d

NormalDd

NormalDd

DDNormal

DdNormal(carrier)

DdNormal(carrier)

ddDeaf

Eggs Sperm

PARENTS

OFFSPRING


• A few are caused by dominant alleles

Figure 9.9B

– Examples: achondroplasia, Huntington’s disease


• Most sex-linked human disorders are due to recessive alleles

– Ex: hemophilia, red-green color blindness

These traits appear mostly in males. Why?

– If a male receives a single X-linked recessive allele from his mother, he will have the disorder; while a female has to receive the allele from both parents to be affected

Sex-linked disorders affect mostly males

Figure 9.23A


Pedigree Chart: Inheritance Pattern for an X-linked Recessive Disease

Figure 19.12


• A high incidence of hemophilia has plagued the royal families of Europe

Figure 9.23B

QueenVictoria

Albert

Alice Louis

Alexandra CzarNicholas IIof Russia

Alexis


Variations on Mendel’s Principles

• Codominance, multiple alleles

• Pleiotropy

• Polygenic traits

• Sex-linked genes

• Environmental effects


• Abnormal chromosome count is a result of nondisjunction

– homologous pairs fail to separate during meiosis I

Accidents during meiosis can alter chromosome number

Figure 8.21A

Nondisjunctionin meiosis I

Normalmeiosis II

Gametes

n + 1 n + 1 n – 1 n – 1

Number of chromosomes


– Or sister chromatids fail to separate during meiosis II

Figure 8.21B

Normalmeiosis I

Nondisjunctionin meiosis II

Gametes

n + 1 n – 1 n n

Number of chromosomes


• An extra chromosome 21 causes Down syndrome

• The chance of having a Down syndrome child goes up with maternal age

Figure 8.20C


• Karyotyping and biochemical tests of fetal cells can help people make reproductive decisions

– Fetal cells can be obtained through amniocentesis

Fetal testing can spot many inherited disorders early in pregnancy

Figure 9.10A

Amnioticfluid

Fetus(14-20weeks)

Placenta

Amnioticfluidwithdrawn

Centrifugation

Fetalcells

Fluid

Uterus Cervix Cell culture

Severalweeks later

Karyotyping

Biochemicaltests


• Chorionic villus sampling is another procedure that obtains fetal cells for karyotyping

Figure 9.10B

Fetus(10-12weeks)

Placenta

Chorionic villi

Suction

Several hourslater

Fetal cells(from chorionic villi)

Karyotyping

Some biochemical

tests


• Examination of the fetus with ultrasound is another helpful technique

Figure 9.10C, D

PGD - Preimplantation Genetic Diagnosisgenetic analysis of embryos from in

vitro fertilization (IVF)before inserting into womb


Genes and Behavior

• Mechanism

– Product from gene-specific proteins

– Proteins have specific functions leading to phenotypes: hormones, enzymes, transport, neurotransmitters

Human Genetics Phenotype: observed physical and functional traits Genotype: complete set of genes...

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