Chapter 12 Inheritance Patterns and Human Genetics

How is your sex determined? Thomas Hunt Morgan ( ) –Won the Nobel Prize in 1933 for work with fruit flies Drosophila & their white-eyed mutation. –Found that Drosophila had 4 pairs of chromosomes with 1 mismatched pair Males XY Females XX. –True for all mammals and most insects Always a 50:50 chance of getting a male or female offspring

Why work with fruit flies? 1.*.5mm long 2.*Fast reproductive cycle Embryo develops 1 day after fertilization to become a larva Pupa 2 days later 4 more days to emerge as an adult 3.Fertile with hours after hatching 4.*Need only a small area to culture many 5.*Reproduce many offspring

Since Y chromosome is so small, could genes be located on the X chromosome that are not on the Y chromosomes? Sex – Linkage – traits that are determined by whether you are male or female Normal fruit flies (wild type) are Red eyed Morgan crossed a white eyed male and a red eyed female fruit fly. All F 1 were wild type fruit flies Did an F 1 cross. –½ the males were white eyed but no females had the white eyes

P: X R X R x X r Y Xr Xr Y XR XR XRXr XRXr XRYXRY X R XRXr XRXr XRYXRY Phenotype ratio: 100% wild type Xr Xr Y XR XR X R

F 1 cross: X R X r x X R Y XR XR Y X R XRXR XRXR X R Y Xr Xr XRXr XRXr XrY XrY F 1 cross: X R X r x X R Y Phenotype ratio: 75% wild type 25% white eyed 100% of females will be wild type 50% of males could be white eyed XR XR Y X R Xr Xr

What cross would yield a white eyed female? X R X r x X r Y Phenotype: 50:50 for males and females for white or wild type eyes Xr Xr Y X R XRXr XRXr X R Y Xr Xr XrXr XrXr XrY XrY

Human Sex Chromosomes The X chromosome is much larger than the Y chromosome. Genes that are located on the top of the X chromosome are not on the Y chromosome Males either have the trait or are normal Females can be carriers for the traits. <>

Linked Genes Genetic linkage is the tendency of alleles found on a chromosome to be inherited together during meiosis Genes nearer to each other are less likely to be separated onto different chromatids during crossover, and are genetically linked The nearer two genes are on a chromosome, the lower is the chance of a swap occurring between them, and the more likely they are to be inherited together.

Chromosome Mapping A genetic map is a map based on the frequencies of recombination between markers during crossover The greater the frequency of recombination between two genetic markers, the further apart they are assumed to be Conversely, the lower the frequency of recombination between the markers, the smaller the physical distance between them.

Sex-linked traits in humans 1.Red-Green color blindness – recessive. Can’t distinquish between colors 2.Muscular dystrophy- recessive – weakens then destroys muscle tissue 3.Hemophilia – recessive – lacks ability to produce clotting factor – bleeders –Ran rampant through the royal families of Europe – “Royal Hemophilia” Queen VictoriaCzar Nicholas II

Detecting Genetic Diseases Amniocentesis The Pedigree

How could you trace a trait throughout a family? Pedigree – shows how a trait is inherited over several generations = Female = Female with the trait = Male = Male with the trait If half colored in, they are a carrier of the trait

Sample Pedigrees

Human Genetic Disorders Color blindness Hemophilia Cystic fibrosis Huntingdon’s disease Tay Sachs Sickle cell anemia

Here is a test for you. Look at the figures below and write down what you see. Do not talk or make any comments during this test!. 4. If you could not see the 29, 45, 56, 6 or 8, you are color blind!! 12 – 20% of the population has this trait.

If normal color vision, you will see a 5. If colorblind, you see a 2 Youtube test

Sex influenced trait Males and females express trait differently with the same phenotype Male Pattern Baldness – recessive – speaks for itself! On autosomes, not sex linked X N X N X N Y N - Normal hair X N X n X N Y n - Normal female, Bald male X n X n X n Y n - Bald

How does the inheritance of traits located on the same chromosome affect how they are inherited? Linkage groups If genes are located on the same chromosome, they tend to be inherited together Morgan - genes for wing length and body color were on the same chrom (linked) Crossing over during Prophase I of Meiosis unlinks linked genes The closer two genes are on a chromosome, the less likely they are to cross over. > probability of crossing over the further away they are on a chrom

Traits controlled by 3 or more alleles for a given trait –Blood type: I A I B i (A, B, O)

Polygenic inheritance 2 or more genes determine a single trait. Ends up blending these genes –Height –Skin pigmentation –Eye color –Hair color

What can go wrong? AKA Mutations 1.Germ cell mutations – affect gametes – may be passed on to offspring 2.Somatic mutations – affect body cells – not inheritable –Some cancers & leukemia

3.Chromosomal mutations – changes in structure of a chromosome or addition or loss of an entire chromosome –Deletion/addition – portion of chrom is missing or added –Inversions – pieces of chroms flip flop –Translocations – pieces of nonhomologous chroms are exchanged A lovely British animation

Nondisjunction – type of Chromosomal mutation – entire chromosomes are missing or extra chromosomes -Chromosomes fail to separate during meiosis -Results in gametes with extra chromosomes or missing chromosomes Animation

Trisomy 21 – Down syndrome –Extra #21 Chromosomes

–If Sex chromosomes fail to disjoin: Turner’s syndrome – X0 female Klinefelter’s syndrome – XXY male

46XX 45X0 47XXY 46XY Descriptions

Disorder Symptom Pattern of Inheritance Frequency at birth Huntington's disease Deterioration of brain tissue - middle age Autosomal dominant 1/10,000 Cystic fibrosis Mucus clogs lungs, liver & pancreas - doesn't make it to adulthood autosomal recessive 1/2,080 (whites) Sickle cell anemia Impaired blood circulation - organ damage autosomal recessive? 1/500 (African decent) Tay-Sachs disease Deterioration of central nervous system - childhood death autosomal recessive 1/1,600 (European Jewish decent) Phenylketonuria Brain failure - doesn't have enzyme for phenylalanine digestion autosomal recessive 1/18,000 Hemophilia Failure to clot blood X-linked recessive 1/7,000 Muscular dystrophy wasting away of muscles X-linked recessive 1/10,000

What to do to help? GENE THERAPY

Other neat stuff… X chromosome inactivation and Barr bodies