Human GeneticsChapter 14 in the Textbook

The gender of an individual is determined by the combination of sex chromosomes inherited.• Females normally have two X

chromosomes.• Males normally have one X and

one Y chromosome.

How are the X and Y chromosomes different in size and number of chromosomes.

Sex-Linked Inheritance

• Make a Punnett square to show the probability of producing a male or female offspring.– Females can only pass on a X chromosome.– Every female egg cell contains 1 X chromosome.– Males can pass on a

X or Y chromosome.– Half of the sperm cell

contain an X chromosome and half contain a Y chromosome.

Sex-Linked Inheritance

Sex-linked Genes: genes located on a sex chromosome

• All genes found on the Y chromosome are passed directly from father to son.

• Males have only 1 X chromosome– All alleles on the X chromosome

are expressed in males, even if they are recessive

– Males are more likely to inherit a trait that is found on X chromosome.

– Many genetic disorder are linked to the X chromosome.

Sex-Linked Inheritance

An example of a sex-linked disorder is colorblindness.• Genes associated with colorblindness are found on the X

chromosome• 1/12 males are colorblind, 1/200 females are colorblind• Draw a Punnett square to show the probability that a

heterozygous normal vision female and a normal vision male would have a colorblind child.

Sex-Linked Inheritance

KaryotypesDifferent organisms have different numbers of chromosomes

• Autosomes: Any chromosome that is not a sex chromosome (X or Y chromosome)– Humans have 22 pairs

autosomes

• Karyotype: a photograph of the complete diploid set of chromosomes grouped in homologous pairs and arranged in order of decreasing size.

How to make a karyotype:1. Photograph a cell going through

mitosis. – Why couldn’t you photograph a cell

in interphase to make a karyotype.2. Match up the chromosomes in

homologous pairs.– What do a pair of homologous

chromosomes have in common?– How are the chromosomes in a

homologous pair different?3. Arrange the chromosomes from

largest to smallest

Karyotypes

• What can be learned from making a karyotype?

– The number of chromosomes in the organism

– The gender– The presence of

chromosomal disorders

• What does the karyotype to the right tell you about the organism?

Karyotypes

• Chromosomal mutation: mutation that changes the number or structure of chromosomes.– Deletion: The loss of all or part

of a chromosome– Duplication: A segment of the

chromosome is repeated– Inversion: part of the

chromosome is reverse from its usual direction.

– Translocation: one chromosome breaks off an attaches to another chromosome.

Chromosomal Disorders

Many chromosomal disorders are caused by a mistake in meiosis called nondisjunction.

• Nondisjunction: homologous chromosomes fail to separate during meiosis– Abnormal number of

chromosomes find their way into gametes

– Causes chromosomal disorders.

– Which of the sperm cells have an extra chromosome, which are missing a chromosome?

Chromosomal Disorders

Disorders caused by nondisjunction.Down Syndrome• Caused by an extra 21st

chromosome.• Also known as trisomy 21• Trisomy: having three

copies of a chromosome.

Chromosomal Disorders

Turner’s Syndrome• Females are born with only one

X chromosome• Monosomy X: having only one

X chromosome.

Klinefelter’s Syndrome• Males born with two X

chromosomes and a Y chromosome.

• Y chromosome determines if the individual is male or female.

Chromosomal Disorders

Pedigree Chart: shows the absence or presence of a trait according to the relationships between family members.

Pedigrees

Determine if the trait is dominant or recessive trait?Determine the genotype of every member in the family.

Pedigrees

What can you learn by making a pedigree:• How genetic disorders can be inherited from one

generation to the next. (sex-linked, autosomal dominant, autosomal recessive)

• If you have a genetic disorder in your family, then pedigree can determine how likely you are to passed the disorder to future generation.

Pedigrees

Cystic Fibrosis• Caused by a recessive allele

on chromosome 7• It is inherited in a autosomal

recessive pattern.• People with cystic fibrosis

lack one amino acid in the CFTR gene, causing a thick mucus that clogs the lungs.

Genetic Disorders

Ideogram: A diagram of a chromosome• Ideograms show can show the

position of specific genes:• The CFTR gene is located at 7q31.2– 7th chromosome– q = long arm of the chromosome (p = short arm of the chromosome)– 31.2 represents the position on the

arm (band number)

Genetic Disorders

Huntington’s Disease• Caused by a dominant allele• The DNA sequence – C A G – is

repeated over several times.• This causes the Huntingin protein

to become deformed which damages the nervous system.

• Most genetic disorders are inherited as a recessive trait. Why are dominant genetic disorders more rare than recessive disorder?

Genetic Disorders

Why does Huntington’s disorder get passed from generation to generation.• Causes gradual damage to the

nervous system resulting in loss of muscle control and mental function.

• People who have the disease show no symptoms until their thirties or forties (after they passed the allele to their children).

Genetic Disorders

Sickle Cell Disease• Characterize by “sickle” shaped red

blood cells• Caused by a mutation of the

hemoglobin gene on chromosome 11.• People who are heterozygous for the

sickle cell trait have both normal and sickle shaped red blood cells.– What is the pattern of inheritance

Genetic Disorders

• Sickle Cell allele is carried by 1 in 12 of African ancestry• Most genetic disorders are extremely rare. Why is sickle

cell so common?– Those who are heterozygous for Sickle cell are

resistant to malaria.– The benefits of being heterozygous outweighed the

risks the of passing on both parents passing the allele for the disease.

Genetic Disorders