Chapter 12 Inheritance Patterns and Human Genetics

Walter Sutton - American Geneticist Developed the Chromosome Theory of Heredity

The Chromosome Theory of Heredity Chromosomes are located in the nucleus Factors (genes) are found on chromosomes Sutton discovered that genes are on chromosomes in 1902

Chromosome Theory of Heredity States that genes are located on chromosomes and each gene occupies a specific place on a chromosome Only one allele is on a chromosome Finally explains what Mendel was talking about

Thomas Hunt Morgan Studied fruit flies – Drosophila melanogaster

Fruit Flies are excellent for genetic studies because: Reproduce quickly Easy to raise Many mutations Have 8 chromosomes (n=4)

Morgan looked at TWO traits Gray bodies – G Normal Wings - W Black bodies – g Small wings – w

P1 GGWW x ggww F1 GgWw 100%

Morgan then mated the F1 back to the recessive parent GgWw x ggww Expected ratio – 1:1:1:1 25% GgWw 25% Ggww 25% ggWw 25% ggww

Morgan’s Actual Results 41.5% gray normal 41.5% black small 8.5 % black normal 8.5% gray small

Conclusion Gene for body size and wing color were somehow connected or linked Can’t undergo independent assortment

Gene Linkage Genes on the same chromosome are linked together Inherited together – THEREFORE they do not undergo independent assortment

Linkage Groups Package of genes that are always inherited together Genes on the same chromosome One linkage group for each homologous pair Fruit flies – 4 linkage groups Humans – 23 linkage groups Corn – 10 linkage groups

So linkage groups explain the high percentages (41.5%) but What about the 8.5%??????

17% had new combinations The combinations that were expected would be: Gray normal – GW or Black small - gw

When they are lined up they can become twisted and switch genes Crossing Over

The 17% that had new combinations are known as Recombinants – individuals with new combinations of genes Crossing Over – gives rise to new combinations – Prophase I

Chromosome Maps The likelihood a crossover will occur that will result in the separation of two genes depends on the distance between the two genes Chromosome maps are diagrams that show the location of genes on a chromosome Two genes separated by crossing- over 1% of the time = one map unit apart Alfred Sturtevant (Morgan’s student) constructed the 1 st chromosome map of fruit flies

NETTIE STEVENS WORKED WITH MEAL WORMS DISCOVERED THAT THEY HAD 20 CHROMOS. MALES HAD 19 REG. SIZE AND 1 SMALL SHE FOUND THE SAME THING IN FRUIT FLIES SHE SAID THE 19 THAT WERE THE SAME WERE AUTOSOMES, & THE MISMATCHED WERE SEX CHROMOS MALES HAVE 1 X AND 1 Y

Sex Chromosomes Nettie Stevens – made observations of meal worm chromosomes

Sex Chromosomes One pair Female – XX Male – XY

Sex Determination 50/50

Genes on Sex Chromosomes Sex chromosomes determine a person’s sex Sex chromosomes also contain other genes

Sex Linked A gene located on a sex chromosome Usually X Example – Fruit Fly Eye Color The gene for eye color is on the X chromosome - not the Y

Fruit Fly Sex Chromosomes XXXY

Females XRXRXRXrXrXrXRXRXRXrXrXr Males XRYXrYXRYXrY Red Eyed White Eyed

Sex-Linked Genetic Disorders Gene for the trait is on the X or Y X has many genes – Y has few Defects easy to spot – appear more in males

Carrier A heterozygous female – has the gene but does not express it – can pass it on to her children

Colorblindness Recessive X linked disorder Cannot distinguish colors Dominant Gene – X C Recessive Gene - X c

Hemophilia Recessive X linked disorder Blood does not clot X H – good gene X h – hemophilia gene

Muscular Dystrophy Results in the progressive wasting away of muscle

Mutations - A change in the DNA of an organism - Can involve an entire chromosome or a single DNA nucleotide - May take place in any cell

Mutations Germ Cell Mutations - Occur in an organism’s germ cells (gametes)- only affect offspring Somatic Mutations - Take place in an organisms body cells and only affect the organism

Mutations Lethal Mutation: Can cause death Often before birth Good Mutations: organisms have a better chance to reproduce have an evolutionary advantage Provide the variation on which natural selection acts

Chromosome Mutations Changes in the structure of a chromosome Loss or addition of an entire chromosome Four Types: duplication deletion inversion translocation

Gene Mutations Point mutation-single nitrogen base is changed Substitutions may not be fatal - there is redundancy in the amino acid codons

Frameshift Mutation When a nucleotide is lost or added so that the remaining codons are grouped incorrectly This can code for the wrong amino acid and create an incorrect protein Insertions and deletions are frameshift mutations

THE FAT CAT ATE THE RAT Deletion causes a frameshift: If you delete the “E” in the THF ATC ATA TET HER AT

Nondisjunction Some chromosome mutations alter the number of chromosomes found in a cell Nondisjunction – the failure of a chromosome to separate from its homologue during meiosis

Nondisjunction Example: Trisomy 21 Nondisjunction also affects sex chromosomes Examples: Klinefelters and Turner’s syndrome

Polyploidy Condition in which an organism has an extra set of chromosomes 3N, 4N Usually fatal in animals Plants – usually more robust Caused by - Nondisjunction

Mutations begin/traits/predictdisorder/

Pedigree Chart A family record that shows how a trait is inherited over several generations. Circles are females, squares are males Blank = Normal Half = Carrier – not shown, pass on Filled = Displays trait

Pedigree chart Female – circleColored - recessive Male – squareEmpty - dominant

Genetic Traits and Disorders Genetic disorders – diseases or debilitating condition that has a genetic basis Single Allele Traits Controlled by a signal allele Usually recessive Example: Hitchhiker’s thumb Multiple Allele Traits Controlled by 3 or more alleles Example: Blood types Polygenic Traits A trait that is controlled by two or more genes Example: Skin color

Huntington’s Disease Caused by a single dominant allele Appears in 30’s or 40’s Progressive loss of muscle control and mental function  death Gene on chromosome 4

Sickle Cell Anemia Caused by a change in one of the polypeptides found in hemoglobin (carries oxygen in RBC) One nucleotide difference Codominant inheritance pattern – H A H S H A - normal allele H S – Sickle Cell allele

Sickle Cell Anemia Sickle Cell Anemia – common in people of African ancestry and from tropical regions Carriers (heterozygous) of Sickle Cell trait (H A H S ) resistant to malaria

Human Blood Groups Multiple alleles – genes with more than two forms Example – ABO and Rh blood groups * Remember – an organism can have two alleles only!

Blood Type Blood Type – determined by the presence or absence of certain things in the blood RBC (Red blood cell) – can carry two different antigens Antigens – molecules that can be recognized by the immune system

Genotypes and Phenotypes ii I A I A or I A i I B I B or I B i I A I B Type O Type A Type B Type AB

II. BLOOD GROUPS-TYPING

DONORS RECEIVERS A’S CAN GIVE TO A’S, AB’S B’S CAN GIVE TO B’S, AB’S AB’S CAN GIVE TO AB’S O’S CAN GIVE TO ANYONE! THEY ARE A UNIVERSAL DONOR A’S CAN RECEIVE FROM A’S & O’S B’S CAN RECEIVE FROM B’S & O’S O’S CAN RECEIVE FROM O’S ONLY! AB’S CAN RECEIVE FROM AB’S, A’S, B’S, & O’S (UNIVERSAL RECIPIENTS)

Rh Blood Groups Rh antigen also on RBC Rh + - have antigen (Dominant) Rh _ - no antigen (Recessive) IF A PREGNANT WOMAN HAS AN Rh+ BABY & SHE IS Rh- THEN THIS COULD BE A PROBLEM IF THE CORD BLOOD MIXES W/THE MOM’S SHE COULD HAVE ANTIBODIES AGAINST THE BABIES BLOOD AND CAUSE DEATH TO THE BABY

Blood Typing Activity er /

Polygenic Inheritance Traits controlled by two or more genes Examples – height, skin color, animal coat patterns Phenotypes are seen in a range

POLYGENIC INHERITANCE INHERITANCE OF SEVERAL GENES TO EXPRESS A SINGLE TRAIT LIKE SKIN COLOR

Sex Influenced Traits A trait that is caused by a gene whose expression differs in males and females Baldness: A sex influenced trait Single gene – two alleles Gene expression is influenced by male hormones

Baldness GenotypeMaleFemale BBNormal bbBald BbBaldNormal

Diagnosis of Genetic Disorders Down’s Syndrome – Trisomy 21 Extra copy of the 21 st chromosome Can be detected by microscopic examinations of chromosomes karyotype

Prenatal Diagnosis Amniocentesis – removes fluid from the sac around the baby The fluid can be used to grow cells and make a karyotype Chorionic Villus Biopsy – cells are removed from the embryo Faster results than amnio

Prenatal Diagnosis CVB and Amnio make it possible to detect chromosomal abnormalities Test for: biochemical abnormalities Presence of certain DNA sequences

Ethics We can detect over 100 disorders Knowledge leads to choices and decisions Ethical Considerations: Do you consider a developing embryo a person? Are you pro-life or pro-choice? Should a person knowingly bring a baby with a deadly disease into the world?