Week 11 Notes
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This 10 page Class Notes was uploaded by Nausheen Zaman on Sunday November 15, 2015. The Class Notes belongs to Bio 1510 at Wayne State University taught by Dr. Nataliya Turchyn in Summer 2015. Since its upload, it has received 67 views. For similar materials see (LS) Bas Life Mch in Biology at Wayne State University.
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Date Created: 11/15/15
Chapter 12 (con) ● Pleiotropy ○ Single gene affects more than one trait (e.g., sickle cell anemia and cystic fibrosis) ● Polygenic Inheritance ○ More than one gene affects a single trait (e.g, human height, eye color and skin color ○ 6 alleles determine eye color (more alleles expressed = darker the color) ● Environment vs. Gene Expression ○ Coat color in Himalayan rabbits and Siamese cats ■ Gene that encodes tyrosinase controlled by change in temp ● Found in the ears, nose and tails of himalayan rabbits and siamese cats ■ In this case, presence/absence of melanin that gives color to skin/hair/fur controlled by tyrosinase whose activity depends on temperature ● When <33C, tyrosinase active → melanin produced → hair/fur turns dark ● When >33C, tyrosinase inactive → melanin not produced → hair/fur stays lighter ● Epistasis ○ One gene affects expression of another gene (coat color in Labs) ■ X = amount of color ● Black (B) dominant allele ● Brown (b) recessive allele ■ Y = whether or not color will be produced ○ Black/Brown labs = produced when dominant allele (E) of gene Y is present ○ Yellow labs = produced when recessive alleles (ee) of gene Y are present ○ Coat color controlled by two different genes ○ E – dominant, e – recessive for coat color presence BLACK BROWN YELLOW/GOLDEN BBEE bbEE bbee BbEe BBee BBEe Bbee BbEE Chapter 13: Chromosomes, Mapping and Meiosis Inheritance Connection ● XLinked Genes ○ 1910 T.H Morgan crossed mutant whiteeyed male to a normal redeyed female ■ All F1 progeny red eyed = dominant trait ( R ) ■ White eyes = recessive trait ( r ) ○ Morgan crossed F1 females with F1 males ○ F2 gen consisted of red and whiteeyed flies ■ But all whiteeyed flies were male ○ Were whiteeyed females viable? He then performed a test cross… ■ The cross between an F1 female and a whiteeyed male showed viability of whiteeyed females ○ Morgan then concluded that the eye color gene resides in the X chromosome ○ It was known at the time that a female fruit fly has XX chromosomes and male has XY chromosomes ○ When X bearing egg fuses with X bearing sperm, then an XX zygote forms and turns into a female ○ When X bearing egg fuses with Y bearing sperm, then an XY zygote forms and turns into a male ○ Xlinked genes – genes that are located on the X chromosome ● What Determines the Sex of an Organism? ○ Sex determination in Drosophila (fruit flies) of X chromosomes ■ Two X chromosomes = female ■ One X and one Y chromosome = male ○ Sex determination in humans basedresence of the Y chromosome ■ Two X chromosomes = female ■ Having a Y chromosome = male ○ Fun fact! Some insects genders are based on number of chromosomes or the number of X chromsomes ■ Male grasshoppers = XO (O = zero, meaning they only have one X chromosome) ● What is ‘Male’ on the Y Chromosome? ○ In humans, the Y chromosome has the sexdetermining region Y (SRY) gene ■ encodes testis determining factor (TDF) protein ■ Swyer syndrome nonfunctional SRY gene ● Y chromosome affected, X chromosome is normal ● Male genotype (XY), female phenotype (no testes, female external genitalia, uterus but no ovaries) ■ People with this syndrome usually sterile ● What is ‘Male’ on the X Chromosome? ○ Males must also produce androgens (e.g. testosterone) and respond to them ■ Androgens male sex hormones ○ X chromosome of males has a gene that encodes androgen receptor ■ Androgen insensitivity syndrome androgen receptor is mutated ● X chromosome is affected, Y chromosome is normal ● Male genotype (XY), female phenotype (no testes) ■ Androgen sensitivity makes men sterile ● SexLinked Disorders ○ Associated with sex chromosomes (X or Y) ■ Xlinked disorders show up in both genders ■ More often in males (only have one X chromosome) ○ Hemophilia (bloodclotting disorder) caused by an Xlinked recessive allele ○ Humans have 25K protein encoded gene (2000 on X, 78 on Y ← these genes involved in sex determination) – nonhomologous ○ Sexlinked disorders associated with these genes ○ Divided into Ylinked and Xlinked ■ Ylinked only found in males (they have the Y chromosome), Xlinked found in both males and females ■ Xlinked disorders more common in males than females (they only have one X chromosome) ○ Human males have only one allele for each gene located on the X chromosome → they cannot be homodominant/recessive or heterozygous for Xlinked disorders, but they can be phenotypically normal or affected ○ Human females CAN be homodominant/recessive or heterozygous because they have two X chromosomes ○ In order for a female to be infected, she should inherit two alleles from her parents ○ Males only inherit one allele from mom to be infected ● Do Women Really Need Two X Chromosomes? ○ In females, cells don’t use both X chromosomes ○ Dosage compensation = One X chromosome is inactivated ○ In each female cell… ■ One X chromosome is inactivated and highly condensed (heterochromatin) into a Barr body ■ Other X chromosome is decondensed and used ○ Human female – two X chromosomes (true for all female mammals) ○ Only one of two X chromosomes is used in female mammals ■ If all genes in both X chromosomes were expressed, too many Xrelated proteins would be released (very dangerous!) ○ One of two X chromosomes becomes Barr body *Barr body doesn’t encode any proteins* ○ Normal human female has one Barr body in each of her somatic cells ○ Normal human male has NO Barr body in their somatic cells ○ Calico cats: ■ Barr bodies form early with amniotic development (random process) ■ Paternal X chromosome turns into Barr body ■ How can a woman be homodom/recessive for Xlinked traits? ● half of alleles in somatic cells = maternal X; half of alleles in somatic cells = paternal X (doesn’t change phenotype in humans) ● Above reason is the cause of calico cat’s fur color ■ Black allele – Dom, Orange allele – Recessive ■ Different colors results in activation of fur alleles in X chromosomes ■ Calico cats hetero for fur located on X chromosome ■ Second gene located on the autosomal chromosome, two different alleles (Dom (S), recessive (s)) ■ Cats: ● Homodominant : more white fur ● Hetero: have different colored patches of fur (calico c ts) ● Homorecessive: only black/orange, but no white ■ Calico cat fur colors due to X chromosome inactivation and epistasis ● Do We Always Inherit One Maternal Chromosome and One Paternal Chromosome? ○ DNA that is commonly referred to = nuclear DNA ■ Inherited from both parents ○ Mitochondrial DNA ■ Found in mitochondria ■ Inherited from mother ○ Both sons and daughters inherit mito DNA ■ Sons cannot pass it on from gen to gen ○ Paternity tests use nuclear DNA ● Problems with the X and Y Chromosomes ○ Nondisjunction failure of homologous chromosomes/sister chromatids to seperate properly during Meiosis I or Meiosis II ■ Example used in the book shows a mistake occurring during Anaphase I or II ○ Aneuploid gametes that have one more/less chromosome ■ Monosomy one chromosome is absent ● Monosomic individual generated when aneuploid gametes have 22 chromosomes fuses with a normal gamete having 23 chromosomes ● has 45 chromosomes in each of their somatic cells ■ Trisomy extra chromosome is present ● Trisomic individual generated when aneuploid gametes having 24 chromosomes fuse with gamete that has 23 chromosomes ● has 47 chromosomes in their somatic cells ● Nondisjunction of Sex Chromosomes ○ XX + Y = XXY male ■ Klinefelter syndrome enlarged breasts, underdeveloped testes ○ O + X = XO female ■ Turner syndrome no breast growth, infertile ○ YY + X = XYY male ○ XX + X = XXX female ■ The above two nondisjunctions have normal phenotypes ○ Y + O = YO zygote ■ Doesn’t survive as there are only 78 chromosomes present in gene ■ All of them determine sex in the zygote and nothing else ● Problems with Autosomal Chromosomes ○ Edwards syndrome trisomy 18 ■ Babies born with this die within a few months ○ Down syndrome trisomy 21 ■ Can survive to adulthood ■ Mother’s age influences risk ○ Both individuals have 47 chromosomes ● How are Genetic Disorders Tested? ○ Amniocentesis ■ Collecting fetal cells from amniotic fluid through a syringe and needle for examination ■ Performed between 1420 weeks of pregnancy ● Amniotic fluid fluid that surrounds the fetus ● water breaking = amniotic fluid ■ Biochem tests reveal whether the baby has sickle cell anemia or not and tests enzymes and other proteins (results found in several weeks) ■ Fetal cells after 23 weeks are used for karyotyping: ● All the cells are dyed ● Each chromosome has a certain pattern that shows when dyed (allows scientists to study chromosomes) ● We can tell the number, gender and length of chromosomes ■ With amniocentesis – risk of miscarriage ○ Chorionic villus sampling ■ Obtains fetal cells from chorionic villi (fingershaped growths found in placenta) for karyotyping ■ Performed between 812 weeks of pregnancy ■ More popular than amniocentesis ● Earlier in pregnancy ● Less invasive (uses suction of tissue sample instead of needle ● Gives faster results than amniocentesis
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