Human Genetics Chapter 14 Sex Chromosomes Autosomal Chromosomes

Human Genetics Chapter 14

Sex Chromosomes Autosomal Chromosomes (Autosomes) Determine the sex of the individual Humans have 2 Females – XX Males – XY the remaining 44 chromosomes that are not sex chromosomes. .

Sex Chromosomes and Sex-linked genes X – Y system : Humans X – 0 system : arachnids and many insects Z – W system : Birds, Fish, some insects like butterflies

Sex – linked trait Alleles - alternate forms of the same gene. Females - Must have 2 alleles to express a recessive trait 1 allele of recessive trait = • carriers of the gene • don’t express the trait Males - one allele express that trait.

Punnett Squares with Sex Chromosomes XH Y XH XH Y Xh XH Xh Xh Y H = normal 1) h = hemophelia % of children with the disorder: 2) % of son’s with the disorder: 3) % of daughters that would be a carrier of the disorder? • 25% • 50%

Pedigree Chart A chart that shows: 1. The genetic relationships within a family 2. How traits are passed from generation to generation.

Blood Types in Humans • ABO Blood Types • A (IA) and B (IB) are codominant • O (i) is recessive

Karyotype a picture of chromosomes arranged in pairs.

CHROMOSOMAL DISORDERS Down Syndrome Turners Syndrome Klinefelter’s Syndrome Swyer Syndrome

Chromosomal Disorders • Nondisjunction • An error in meiosis when homologous chromosomes fail to separate • Abnormal numbers of chromosomes in gametes

Down Syndrome 3 copies of chromosome 21 Mental disability and wide set eyes 1/800 babies born with Down Syndrome in the US Trisomy – end up with 3 copies of a chromosome How Down Syndrome Occurs

Turners Syndrome Only one X chromosome (only 45 chromosomes) Sterile females, short in stature, limited development at puberty, limited to no secondary sex features (breasts etc. ) Hard to detect. Monosomy - end up with only 1 chromosome. How Turners Syndrome Occurs

Klinefelter’s Syndrome • Develop as males. • Males with this disease inherit an extra X chromosome (anywhere from 1 to 4 extra X’s). • Males with this disease are sterile and have female characteristics such as breasts. • How Klinefelters Syndrome Occurs

Swyer Syndrome & the SRY Gene A gene located on the Y chromosome. If this gene is not present, the child will develop as a female even if she has the Genotype XY. Female will have masculine features and will be infertile Normal male develompment: The SRY gene activates the testes, which in turn produce testosterone. Without SRY Gene: the testes never develop, and testosterone is not released causing the genotypical male to be a female.

AUTOSOMAL DISORDERS Albinism Cystic Fibrosis PKU Tay-Sachs Disease Huntington’s Disease Sickle Cell Anemia

Autosomal Genetic Disorders • Autosomal Dominant • Huntingtons Disease and others • Autosomal Recessive • Cystic Fibrosis, Sickle Cell Anemia, Tay-Sachs, Spinal Muscular Atrophy • Autosomal Codominant • Sickle Cell Anemia

Autosomal Recessive Alleles Both parents carry one normal gene and one altered gene. What’s the chance that a child will end up with the disorder? What’s the chance that a child – 25% will end up being normal? What’s the chance that a child will end up being a carrier of the - 25 % disorder? – 50%

Tay Sachs Disease: Lipid accumulation in brain cells results in nervous system breakdown & death within a few years of life. Death within 1 -3 years of age. 1/27 born (linked to Jewish decent)

Cystic Fibrosis • Cystic Fibrosis: • Increased infections due to an excess of liquid in the lungs and digestive tract • Early death is common. • 1 in 25 people born w/c. f.

PKU – phenylketonuria Lack enzyme to break down phenylalanine which is found in milk. Can cause mental disabilities, organ damage and unusual posture. Can test at Birth With careful dietary supervision, children born with PKU can lead normal lives, and mothers who have the disease can produce healthy children. Caused by a recessive allele on chromosome 12. 1 in 15, 000 born with PKU

• Albinism • Little to no pigment in eyes, skin, or hair due to lack of melanin • 1 in 17, 000 in the USA have some type of albinism.

Autosomal Dominant Alleles These genes can be inherited from either parent. How many dominant alleles must be present for the individual to express that gene in their phenotype? Only one Always expressed if that allele is present

Dwarfism • Achondroplasia (Dwarfism)

Huntington’s Disease • progressive loss of muscle control and mental retardation • Symptoms occur when people reach their 30’s

Codominant Autosomal • Codominant Alleles • Both dominant alleles are fully expressed.

Sickle Cell Anemia • Disorder common in African Americans • Abnormal (sickle) shape of cells • This causes them to get stuck in capillaries blocking blood. • Just a single DNA base change results in this disease. • Offers some resistance to malaria parasite. • Lack of oxygen to tissues damages tissues.

SEX LINKED DISORDERS

Sex-Linked Genes Caused by genes located on x & y chromosomes Males have just one X chromosome, thus all Xlinked alleles are expressed in males, even if they are recessive. Females For a recessive allele to be expressed in females, how many copies of the allele must be present? 2

Sex Linked Dominant Recessive

X-Linked Genetic Disorders • X-Linked Dominant • X-Linked Recessive • Hemophelia, Muscular Dystrophy, Color Blindness, Duchenne Muscular Dystrophy

Colorblindness • inability to see color or certain colors • Found on the X chromosome. • Red-green most common form. • 1/10 females & 1/100 males in US Everyone Should See This One

Hemophilia • Inability to clot blood normally due to lack of necessary protein. • Individuals could bleed to death from even a minor cut. • Found on the X chromosome. • 1/10, 000 males

Muscular Dystrophy • Duchene Muscular Dystrophy • the progressive weakening and loss of skeletal muscle leading to death in early adulthood. • Found on the X chromosome.

Y-Linked Genetic Disorders • Y-Linked • Male Infertility

CHROMOSOME INACTIVATION

X-Chromosome Inactivation In X-chromosome inactivation in females, one of the X chromosomes is inactivated. It condenses in the nucleus and is called a Barr body. Calico cats are female only. Gene for orange fur is on one X and the gene for black is on the other X. Some regions of the cat’s body have the X chromosome with the gene for black fur inactivated while other regions have the orange color inactivated.

Genetic Testing • Testing for alleles • parents can have this done to identify genes that may cause disorders in their children.

Gene Therapy • An absent or faulty gene is replaced by a normal, working gene. • This is an option for parents who go through allele testing so that they can avoid having children with disorders. • At this point in time, gene therapy is a high-risk experimental disorder, but could be very useful in the future.

Human Genome Project • An attempt to sequence all human DNA (began in 1990 and was complete by 2000 • Rapid Sequencing – used computers to piece together fragments of DNA until the entire genome was mapped out. • Humans – 31, 000 genes • A Breakthrough for Everyone – all data is available on the Internet.

Ethical Issues • It is important for us as a society to decide what is ethical and what is not. • Is it ethical to allow parents to change a gene if they know that their child is going to be born with a disorder such as Tay Sachs and will die by the age of 3? • Is it ethical to allow parents to change characteristics of an individual such as eye color and hair color?