Human Heredity

Learning Target 14.1

Explain how human traits are inherited.

Explain why human traits are not ideal for the study of genetics.

Discuss the influence of the environment on gene expression.

Studying Genetics 100 years ago Thomas Hunt Morgan

proposed using the fruit fly Drysophyla melanogaster as a way of studying genetics for three reasons.

1. They reproduce quickly (2 weeks)2. They are small and can be kept in large numbers3. They only have 8 chromosomes (4 pair)

which are relatively large and easily seen.

                                        

                                            

Here is what is done! The human organism in contrast contains 23

pairs, or 46 chromosomes To study these biologists must trap cells

in the metaphase stage of mitosis using a poison that destroys microtubules.

From here they take a picture, cut out the chromosomes and pair them up in what’s known as a karyotype.

A karyotype reveals there are two sex chromosomes, X and Y and 44 autosomes

Karyotype

Normal female

Normal Male

Reproductive Cells

Human sex cell contains 22 autosomes and 1 sex chromosome

The egg and sperm unite to form a zygote

During meiosis females will produce an egg with an X chromosome and males will produce sperm with an X or Y.

Pedigree Analysis

Human generations consist of about 20 years which makes most experiments impossible to carry out.

A Pedigree analysis makes it possible to study several generations of traits

Pedigree

Genes and People

Many human traits are inherited by the action of dominant and recessive genes, although other traits are determined through more complicated gene interactions

Human Blood Groups

Multiple alleles are three or more alleles of the same gene that code for a single trait. ABO and RH blood groups are

determined by multiple alleles.   In 1900 an Australian physician Karl

Landsteiner discovered that there were four types of blood groups

Blood Types

Individuals with AB Blood Produce both

antigens Blood type is

Individuals with AA or AO Produce only A

antigens Blood type is

Individuals with BB or BO Produce only B

antigens Blood type is

Individuals with OO Do not produce

antigens blood type is

Human Blood Groups

Blood type exists as four possible phenotypes There are 2 main genes responsible for blood

type ABO Blood Groups – determined by single gene

with 3 alleles (MULTIPLE ALLELES) Rh Blood Groups – determined by single gene

with 2 alleles Rh+ allele is dominant over Rh- allele What is phenotype of someone who is Rh

positive? What is phenotype of someone who is Rh

negative?

ABO Blood Group

The three alleles are: IA (we will write as A)- Codes for “A” blood IB (we will write as B)- Codes for “B” blood i – recessive allele (we will write as O)- Codes

for “O” blood

ABO Blood Group

Alleles A and B are codominant These alleles produce antigens that can

be recognized by immune system on the surface of the blood cells

The allele for O is recessive to the alleles A and B Individuals with ii (or O blood type)

produce NO antigens

Universal Recipient and Donor

Universal Donor – Blood Type O Has NO antigens

Universal Recipient – Blood Type AB Has BOTH antigens

SAFE TRANSFUSIONANTIGENPHENOTYPE

TO FROMGENOTYPE

Referring to a Blood Type

When you refer to a blood group, you use both groups at the same time EX: Person with AB- Blood Type

Individual has AB alleles from the ABO Gene Individual has Rh- allele from the Rh gene

Lets try it together:

Suppose a mother who is heterozygous for A blood type has a child with a father

who is blood type O. What would their potential children's blood types be?

When they had they had their first born child there was a bit of a mix up at the hospital and they are not sure that they brought home the right baby. The baby they brought home is homozygous A. Is

this their child??

Learning Targets 14.2

How is sex determined? How do small changes in DNA cause

genetic disorders. Why are sex-linked disorders more

common in males than in females?

Sex Chromosomes

In the early part of the nineteenth century Nettie Stevens discovered the sex chromosomes X and Y when studying the meal worm.

It was later discovered that humans displayed the same system. It is the presence or absence of the Y chromosome that determines the sex of an individual

Sex Chromosomes

•In 1909 Thomas Hunt Morgan did an experiment on fruit flies to see how white eyes were inherited.

•He did a cross between a white eyed male and a red eyed female and found all were red eyed.

XRXr

Xr Y

XR

XR XRXr

XRY

XRY

XRXR

XR Y

XR

Xr XRXr

XRY

XrY

•When he did the F2 cross he found ¼ white eyed flies like he expected however he noticed all the white eyed flies were male.

•Morgan had found Sex-linked (X linked) genes.

Sex Linked Genes

Sex Linked Genes — found on sex chromosome Ex: Colorblindness, Hemophilia, Duchenne

Muscular Distrophy Many sex-linked genes are found on the X

chromosome More than 100 sex-linked genes are on the X

chromosome Y Is much smaller and appears to contain few genes

Since males have only one X chromosome, all X-linked alleles are expressed

So, sex linked diseases are more common in males

Sex linked genes move from Mothers to Sons

In females, there is an extra X, so one X is randomly switched “off” The “off” X forms a Barr bodyBarr body – just a

dense region of nucleus

Sex Linked Genes

Examples of Sex-Linked Disorders Colorblindness

Three genes associated with color vision are on X A defective version of any of these produces

colorblindness in males Red-green is the most common form, and appears in

1 in 10 males in the US In Females, it is rare—1 in 100 females has

colorblindness

WHY IS IT DIFFERENT IN MALES & FEMALES? Colorblindness is recessive

Females must have two copies of allele to have disorder Males have just one X and only need one copy

Father(normal vision)

Colorblind Normal vision

Mother (carrier)

Daughter(normal vision)

Son(normal vision)

Daughter(carrier)

Son(colorblind)

Male

Female

Colorblindness

Examples of Sex-Linked DisordersHemophilia

Individuals with the recessive alleles for hemophilia are unable to clot blood properly.

It affects about 1 in 10,000 males and 1 in 1 million females.

Examples of Sex-Linked Disorders

Duchenne Muscular Dystrophy It affects 1 in 3,000 males who suffer a

sudden weakness in muscles that eventually leads to death

Autosomal Genetic Disorders• Although many genetic disorders are

located on the X chromosome, the majority are located on autosomes.

• Autosomal Disorders – found on the autosomes #1-22

• Ex: Albinism, Cystic Fibrosis, Tay Sacks, Sickle Cell Anemia, Huntingtons Disease

Types of Autosomal Disorders Albinism

Albinism is a condition in which the skin is unable to produce melanin a skin pigment.

Albinism is caused by a recessive allele on chromosome 11.

Types of Autosomal DisordersCystic fibrosis  Cystic fibrosis is the most common fatal

genetic disease. It affects people of European ancestry

affecting 1 in every 2500 people. Cystic fibrosis is found on chromosome

7 and causes a build up of liquid in the lungs

Types of Autosomal DisordersTay-Sacks Disease Tay-Sacks disease is a fatal genetic

disorder caused by a recessive allele. It affects people of Jewish ancestry. People affected by Tay-Sacks suffer a

rapid breakdown of the nervous system.

Types of Autosomal Disorders Sickle Cell Anemia

The condition causes many of the blood cells to be sickle shaped.

A person with sickle cell anemia is easily deprived of oxygen and the affected blood cells often become lodged in capillaries which causes serious damage or death.

•The gene for sickle cell anemia (S) is codominant with the normal hemoglobin gene (A).

• People who are heterozygous are carriers and about half of their blood cells are affected. 

• These people suffer few ill effects.S A

A

S S S

S A

S A

A A

Types of Autosomal Disorders People who are homozygous SS are

sufferers and are severely affected because all their blood cells are sickle shaped.

* People of African ancestry are the most common carriers of sickle cell anemia.

About 10% of people in USA are carriers but as many as 40% are carriers in some African countries.

The reason it is so common even though it is detrimental is that people who are carriers are partially resistant to malaria.

  Those who are normal are not resistant and those who have sickle cell usually don't reproduce.

This causes carriers to be more common in some regions.

Types of Autosomal DisordersHuntington Disease Huntington disease, which is produced

by a single dominant allele (H), is an example of a genetic disease.

Huntington disease is a disease that does not express itself until a person reaches their thirties or forties.

Types of Autosomal Disorders The disease causes a painful

progressive loss of muscle control and mental function until death occurs.

Because the disease doesn't express itself until later in life, it is often passed on to the next generation.

 

Polygenic traits

Human traits controlled by a number of genes are called polygenic.

Example; height, weight, and skin color

Chromosomal Disorders NONDISJUNCTION: Homologous chromosomes fail

to separate during meiosis (literally means “not coming apart”)

Possible wrong number of chromosomes in gametes

Possibly resulting in the wrong number of chromosomes in offspring

Homologous chromosomes fail to separate

Sex Chromosomal Disorders

The two most common nondisjunction disorders include: Turners Syndrome Klinefelter Syndrome

In females, Turner’s Syndrome- Girls who only get one X chromosome

Genotype: XO Abbreviated 45XO and are sterile

Sex Chromosomal Disorders

In males, Klinefelter’s syndrome Boys who get extra X’s

Genotype XXY, XXXY, XXXXY

Note: No reports of babies born without an X (in other words, with just a Y) X is ESSENTIAL for development

Prenatal Diagnosis

Prenatal Diagnosis Down syndrome and other genetic

disorders can be detected by analyzing cells from the developing embryo.

 Amniocentesis is a technique in which some fluid is taken from the sac around the embryo

Prenatal Diagnosis

The cells are then grown and treated with a chemical that prevents division and the cells are then broken and the chromosomes are stained and studied.

* A karyotype is then made which displays all the chromosomes so they can be studied

Prenatal Diagnosis

Chorionic villus biopsy (biopsy of the placenta) is an alternative to amniocentesis.

This test can be performed between (10-12 weeks) which is earlier than the amneo

Human Genome Project (add to study guide)

A research initiative began in 1990 with a purpose of: To analyze human DNA sequence Identify all 20,000 – 25,000 genes in human

DNA and their location in the genome

Also… Address ethical, legal and social issues that arise as a result of the project

FINALLY completed in 2003 (13 year long project)