Overview of Human Genetic Disorders

Chapter 14 The Human Genome

List some human genetic disorders that result from chromosome inheritance

Describe a karyotype and explain how it is used to diagnose chromosome problems

Describe the kinds of chromosome mutations that can occur as a result of nondisjuntion

Give an example of a disorder caused by nondisjunction

2 Major Categories of Disorders

• Chromosome Disorders (Ch. 10)

– Result from chromosome segregation problems during meiosis

• Inherited Gene Disorders (Ch. 11)

– Based on inheritance of alleles

Let’s Review / Recall Info

• What is the diploid chromosome number of normal human cells?

• Meiosis produces what kinds of cell?

• How many cells are formed by the end of meiosis and what is their chromosome number?

So…What happens if chromosomes don’t separate during

Anaphase 1

– Homologous pairs

don’t separate

properly

Anaphase 2

– Sister chromatids

don’t separate

properly

Let’s look at some examples

Nondisjunction

Failure of chromosomes to separate during

meiosis

Causes change in normal chromosome

number of gametes and subsequently of

the zygote

Nondisjunction: Meiosis Gone Wrong

• Three Common Results

1) TRISOMY: An extra chromosome ends up in a

gamete and the zygote has 47 chromosomes

2) MONOSOMY : A gamete ends up with a missing

chromosome and the zygote has 45 chromosomes

3) ARM DELETION: loss of the “p” arm or the “q” arm,

still have 46 chromosomes but genes are missing

Keep in mind there are lots of other scenarios!!

Arm Deletion

• Chromosomes have two regions “arms”

– P arm = Petite arm, short arm

– Q arm = longer arm

– Loss of either arm can cause seriousproblems

#5 – p; Cri-du-Chat

#11 – p; Wilm’s Tumor

How do you detect Chromosome Disorders?

• Karyotypes– Image of chromosomes taken from a metaphase cell

– Can be used to analyze chromosome number

Chromosome Characteristics

4 General Characteristics Used to

Identify Each Chromosome

1. Chromosome Length

2. Centromere Position

3. Banding Pattern

4. Satellite Endings

Overview of Human Genetic Disorders

Chapter 14 The Human Genome

Distinguish between gene inheritance and chromosomal mutations that can lead to human traits

Compare the four major types of inheritance patterns observed in humans and give an examples of each:

autosomal dominant & recessive, sex-linked co-dominant & multiple alleles

2 Major Categories of Disorders

• Chromosome Disorders (Ch. 10)

– Result from chromosome segregation problems during meiosis

• Inherited Gene Disorders (Ch. 11)

– Based on inheritance of alleles

Gene Based Human Disorders

• Recessive Allele Disorders

– Albinism

– Cystic Fibrosis

– Galactosemia

– Phenyketonuria (PKU)

– Tay-Sachs Disease

• How many copies of the allele do you need to have to show the disorder?

Gene Based Human Disorders

• Dominant Allele Disorders

– Achondroplasia

– Huntington Disease

– Hypercholesterolemia

• How many copies of the allele do you need to have to show the disorder?

Gene Based Human Disorders

• Co-Dominant Allele Disorders

– Sickle Cell Anemia

• How many genotypes?

• How many phenotypes can occur?

Gene Based Human Disorders

• Sex-Linked Disorders

– Color-blindness

– Hemophilia

– Duchenne Muscular Distrophy

• X chromosome – How many genes?

• Y chromosome – How many genes?

Sex-Linked Disorders

• Genes located on sex chromosomes are said to be sex-linked

– more genes on the X chromosome

– cannot survive without the X chromosome

– most disorders are recessive

• What does this mean for males?

Sex Linked Inheritance

4 Sex-Linked Traits:

Normal Color Vision:A: 29, B: 45, C: --, D: 26

Red-Green Color-Blind:A: 70, B: --, C: 5, D: --

Red Color-blind:A: 70, B: --, C: 5, D: 6

Green Color-Blind:A: 70, B: --, C: 5, D: 2

Sex-Linked Punnett Squares

• Let’s do some practice.

• Punnett Squares are constructed just like all the others we have done, with one exception

• We will only use the XX chromosomes from the mother, and the XY chromosomes from the father

Inheritance Of Color Blindness In Men & Women

Sex-Linked Gene on the X Chromosome:

+ = Normal Vision (Dominant)

o = Color Blindness (Recessive)

Sex Color-blind Normal Vision

Male XoY X+Y

Female XoXo X+X+ X+Xo

Cross Between A Color-blind Man (XoY) andHeterozygous Normal Vision Woman (X+Xo)

Gametes Xo Y

X+ X+Xo X+Y

Xo XoXo XoY

In the above cross, four different possible offspring are produced:

XoY: Color-blind Boy (1/4 or 25%)X+Y Normal Vision Boy (1/4 or 25%)XoXo Color-blind Girl (1/4 or 25%)X+Xo Heterozygous Normal Vision Girl (1/4 or 25%)

Let’s Practice Sex Linked Punnett Squares

Ch. 14 Human Genetics

Essential Question: How are genes inherited in human populations?

Blood Background

4 – 5 L of blood in a person.

Blood is made up of:a solid portion – cells – RBC, WBC and platelets.and a liquid portion – plasma.

Plasma – mostly water- acts as buffer to prevent changes in pH

Blood Types – A, B, AB, O

Name of blood type is based on the name of the antigen (surface molecule) on the surface of the RBC. The gene is carried on chromosome 9.

Type A – RBC contains antigen A

Type B – RBC contains antigen B

Type AB – RBC contains BOTH antigen A and antigen B

Type O – RBC contains No antigens

People produce antibodies against antigens they do not have.

• Three alleles are involved in determining blood type IA, IB, and i

Any two of the three alleles may be combined.

Both IA and IB are dominant and show codominance if inherited together

i is recessive to both dominant alleles

Inheritance of 2 recessive alleles results in type O blood.

Rh factor – is an additional antigen that can be found on red blood cells

It determines whether or not your blood type is positive or negative.

Rh+ - person has antigen = dominant alleleRh- - person does not have antigen = recessive allele

Combine Blood Antigen and Rh factor – It determines whether or not your blood type is positive or negative. This is a dihybrid cross – two different genes showing up at the same time.

A- or A+ B- or B+ AB- or AB+ O- or O+

O positive is the most common blood type. Not all ethnic groups have the same mix of blood types.

BloodType

CaucasiansAfrican

AmericanHispanic Asian

O + 37% 47% 53% 39%

O - 8% 4% 4% 1%

A + 33% 24% 29% 27%

A - 7% 2% 2% 0.5%

B + 9% 18% 9% 25%

B - 2% 1% 1% 0.4%

AB + 3% 4% 2% 7%

AB - 1% 0.3% 0.2% 0.1%

Blood Types and Populations

Riddle me this…

The universal red cell donor has Type O negative blood type. Why are they red cell donors?

The universal plasma donor has Type AB positive blood type. Why are they plasma donors?

ABO Blood Types in Humans

• The alleles for determining – the ABO blood type are a

combination of codominance and multiple allele patterns of inheritance

– Rh factor is based on simple dominance and recessive inheritance

• Determine offspring genotypes using a dihybrid cross

Practice Problems

• A mother who is homozygous for type A blood and heterozygous for Rh factor marries a man who is heterozygous for type B blood and homozygous for Rh+

factor. What is the probability they will have a child who is AB+? Remember that Rh+ is dominant to Rh-

A A

B AB AB

O AO AO

Probability of AB+ :

Type AB and Rh+

2/4 X 4/4 = 8/16

or 1/2

+ -

+ ++ +-

+ ++ +-

Erythroblastosis Fetalis

If mother is Rh- she will produce antibodies againstRh+ antigen if blood cells of baby enter her body.Blood doesn’t usuallymix until birth so first baby is OK.

If next baby is Rh+, mother’s antibodies may cross into baby’s Bloodstream and destroy baby’s RBC

For More Information

• http://learn.genetics.utah.edu/content/begin/traits/blood/transfusions.html

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