Notes Genetics Mendel Human Genetics HonorsGifted Genetics Heredity

Notes: Genetics Mendel & Human Genetics (Honors/Gifted)

Genetics & Heredity w What is genetics? n the science of heredity w What is heredity? n passing of traits from parents to offspring

Gregor Mendel w Austrian Monk interested in inheritance w Studied offspring from different “matings, ” or crosses, of pea plants. w He began his research with pea plants that were considered true-breeding. l Plants that have the ability to produce offspring identical to themselves, through the use of their own gametes = “Self-Pollination” = true breeding

Gregor Mendel w w w Most plants reproduce via cross-pollination, the combining of gametes from two separate plants. Mendel DID NOT want to have this extra variable in his research, so he prevented his plants from “self-pollinating” by crossing the pea plants himself. He began his research looking at 7 different pea plant TRAITS § a characteristic that varies from one organism to another

Mendel’s Research w All 7 traits Mendel studied occurred in one of 2 forms. w He cross-pollinated plants with different forms of the same trait (i. e. purple x white) to examine offspring.

After crossing these plants Mendel discovered that… 1. Inheritance is determined by genes. l Segments of DNA that determine a specific trait 2. Some alleles (different forms of a gene) are considered Dominant and some are Recessive: The Principle of Dominance. l l Dominant alleles that are expressed (usually capital letters) Recessive alleles that are hidden (usually lower-case)

His Experiments w Mendel conducted Monohybrid Crosses (mating involving only one pair of traits). n He cross-pollinated 2 pea plants with opposing traits n The original mating pair he used was called the Parental or P – Generation. l Ex Purple flowering plant & white flowering plant

Mendel - Monohybrid Crosses n n The offspring of this Parental cross is called the F 1 Generation, meaning “first filial. ” Results: ALL PLANTS’ FLOWERS WERE PURPLE!

Mendel – Monohybrid Crosses w Mendel’s 2 nd Experiment… n Mendel allowed the F 1 Generation (all purple) to self-pollinate. w Results: The F 2 Generation (“second filial”) produced flowers in a ratio of 3 purple: 1 white w This 3: 1 ratio occurred for each of the 7 traits in the F 2 Generations!

Alas…Mendel’s Conclusions 1) Every inherited trait has 2 copies of the gene – one from each parent. 2) There alternative versions of genes (alleles). 3) When 2 different alleles occur together, one can be completely expressed (dominant) while the other can be hidden (recessive). 4) Gametes (sperm and eggs) each carry one allele for a given trait; during fertilization, the offspring receives 1 allele from each parent.

Now let’s review some basic Terms… w Gene: Segment of DNA that determines a specific trait w Trait: a characteristic that varies from one organism to another w Allele: Different forms of a gene w Dominant Allele: an allele that hides a recessive trait; usually characterized by a capital letter. w Recessive Allele: an allele that can be “masked” or hidden by a dominant allele; usually characterized by a lower-case letter

Homozygous vs. Heterozygous w Homozygous: Offspring have two identical alleles (PP = homozygous dominant; pp = homozygous recessive); also known as “pure-bred” w Heterozygous: Offspring has one of each allele (Pp = heterozygous); also known as “hybrid”

Genotype v. Phenotypes w Genotype: the set/ combination of alleles an organism has for a certain trait…the “letters” (ex: PP, pp, or Pp) w Phenotype: the physical appearance of a trait in an organism…. the “looks” (ex: blonde hair) Genotype Phenotype

The Law of Segregation w 2 alleles for a trait segregate (separate) when gametes are formed; each offspring receives one trait from their parents

The Law of Independent Assortment w Alleles for different genes separate independently of each other during gamete formation. w One trait, like flower color, does not influence the inheritance of another trait, like plant height.

Probability & Genetics w Probability = the likelihood that a particular event will occur. w Principles of probability can be used to predict the outcomes of genetic crosses. w The more trials conducted, the closer the result will come to the EXPECTED ratio. w The Punnett square can predict the “probability” of outcomes resulting from a genetic cross.

Predicting Results: Punnett Squares w Punnett Squares are diagrams that use the Laws of Segregation and Independent Assortment to predict offspring w Possible gametes for 1 parent are placed along the top of the square; possible gametes for the other parent are written on the left of the square. w The genotypes are predicted by combining alleles R r from each parent. R RR Rr rr

For example… w We can use ratios to express genotypes and phenotypes. n Genotypic Ratio = ___ YY: ____ Yy : ___yy n Phenotypic Ratio = ____ yellow : _____ green

Predicting Results: Punnett Squares w Genotypic Ratio = 0 YY : 4 Yy : 0 yy 4 Yy or 100% Yy w Phenotypic Ratio = 4 yellow : 0 green 100% yellow

Predicting Results: Punnett Squares w Dihybrid crosses (crosses involving 2 traits) are a little more complex. w Possible combinations for the different types of alleles are placed at the top and sides of the square. w Example: (Round, Yellow) (wrinkled, green) Rr. Yy rryy Possible allele combos RY Ry r. Y ry ry Rr. Yy Rryy rr. Yy rryy

BEYOND DOMINANT & RECESSIVE ALLELES Part II:

Beyond Dominant & Recessive Alleles… w Some traits are Polygenic, meaning that more than one gene determine the phenotype. n Examples human hair color, eye color, weight, skin color

Incomplete Dominance w One form of a trait is NOT dominant or recessive to the other. Results in an “in-between” phenotype three different phenotypes are possible. w Example Four O’Clocks flower Red (RR) X White (WW) Pink (RW)

Co-dominance w When two dominant alleles are expressed at the same time in an offspring n Example: A homozygous red horse mates with a homozygous white horse to produce a horse with BOTH red and white hair (called a Roan coat).

BLOOD TYPES Part III:

Blood Types: A Result of Multiple Alleles – Genes with 3 or more possible alleles determining a trait n n Each individual receives only 2 alleles, but there are more than 2 in the population Example = Blood Type w Blood Phenotypes = A, B, AB, and O w Blood Alleles = IA, IB, and i w IA and IB are both dominant (codominant when together), & i is recessive

Blood Typing Chart Blood Type Possible Genotypes Can DONATE A ; AB Can RECEIVE Type A I AI A I Ai Type B IB IB I Ai B ; AB B; O Type AB I AI B AB Universal RECEIVER Type O ii Universal DONOR O A; O

Blood Type Frequency in Population

HUMAN GENETICS Part IV:

Human Genetics w Sex Chromosomes The two chromosomes that determine an individuals sex are XX (mom) or XY (dad). w Autosomal Chromosomes The other 44 chromosomes, not sex chromosomes w Karyotype = a picture of chromosomes arranged in 23 matching pairs; Sex chromosomes are ALWAYS on pair #23.

2 Types of Disorders w Sex-Linked Disorders w Chromosomal Disorders ***But first, let’s talk a little about Sex. Linked Traits…***

Sex-Linked Traits w All eggs carry an X chromosome w Females are XX; Males carry XY w In females, if a defective gene rides on one of the X chromosomes, the other X is likely to have a good copy of the gene that can take over for the “bad” gene w Males do not carry the backup copy of the X chromosome, so the gene is expressed

Sex-Linked Disorders w The information/ traits on the sex chromosomes are called sex-linked genes. Because these chromosomes determine sex, disorders caused by these genes are sex-linked disorders. Typically, these genes are found on the X chromosome. w Examples n n n Colorblindness Hemophilia Duchene Muscular Dystrophy

Chromosomal Disorders w Nondisjunction (failure of homologous chromosomes to separate) occurs during meiosis. The resulting individual has an abnormal number of chromosomes and that results in a disorder! w Examples: n n n Down Syndrome (Trisomy 21) Turner’s Syndrome (XO) sterile Klinefelter’s Syndrome (XXY)