What determines are phenotypes? Autosomes- chromosomes 1-44, pairs 1-22 Sex chromosomes- 23 rd pair of chromosomes – Females have two copies of a large X chromosome – Males have one X and one small Y chromosome All the traits Mendel studied were determined by genes on autosomes Most traits are determined by autosomal genes Many genetic disorders are caused by autosomal genes

Disorders caused by recessive alleles – Cystic fibrosis- severe recessive disorder that mainly affects the sweat glands and mucus glands – Carrier- does not show disease symptoms, but can pass on the disease-causing allele to offspring Disorders caused by dominant alleles- far less common – Huntington’s disease- damages the nervous system and usually appears during adulthood

Some alleles are neither dominant nor recessive, and many traits are controlled by multiple alleles or multiple genes Incomplete dominance- one allele is not completely dominant over another – Red plants (RR) crossed with white plants (WW)- pink (RW) Codominance- both alleles contribute to the phenotype – Black feathered chicken (BB) crossed with white (WW)- all speckled offspring (BBWW) – ABO blood types- also considered multiple-allele trait Polygenic traits- traits controlled by two or more genes – Variation in skin color – Eye color Can the environment affect the phenotype?

Sex-linked genes- genes found on the sex chromosomes Many sex linked genes found on X chromosome Y chromosome is much smaller than X, contains only a few genes Colorblindness- an inability to distinguish certain colors, most common is red-green, more common in males Sex linked disorders are more common in men – X-linked alleles are always expressed in males, because males have only one X chromosome

Hemophilia- another sex linked disorder, a protein necessary for normal blood clotting is missing Muscular Dystrophy- sex linked disorder, results in progressive weakening and loss of skeletal muscle

Pedigree- chart for tracing genes in a family – Trace phenotypes and genotypes – Boxes- represent males – Circles- represent females – If phenotype is more common in males, the gene is likely sex- linked

Karyotype- picture of all chromosomes in a cell Can show changes in chromosomes – Deletion of part of a chromosome or loss of a chromosome – Large changes in chromosomes – Extra chromosomes or duplication of part of a chromosome

Nondisjunction- homologous chromosomes fail to separate during meiosis – May involve autosomes, sex chromosomes, homologous chromosomes If nondisjunction occurs, abnormal numbers of chromosomes may find their way into gametes, and a disorder of chromosome numbers may result Trisomy- when two copies of autosomal chromosomes fail to separate during meiosis, individual is born with three copies of chromosomes Down syndrome- most common form of trisomy, involves three copies of chromosome 21 – Produces mild to severe mental retardation, and higher frequency of some birth defects

Turner’s syndrome- females only inherit one X chromosome – Are sterile- unable to reproduce – Sex organs do not develop at puberty Klinefelter’s syndrome- males inherit an extra X chromosome – Prevent individuals from reproducing No reported instances of babies born without an X chromosome – Contains genes vital for survival and development of embryo