Human Genetics Chapter 14 in the Textbook

Sex-Linked Inheritance 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.

Karyotypes Different 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.

Karyotypes How to make a karyotype: Photograph a cell going through mitosis. Why couldn’t you photograph a cell in interphase to make a karyotype. 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? 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?

Chromosomal Disorders 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.

Pedigrees 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.

Genetic Disorders 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.