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CHAPTER 9 Mendel and the Gene Idea… and a little bit about human genetics.
What is INHERITANCE? The study of how physical traits are passed from generation to generation. How are they passed on???
GENES within DNA. • DNA is the UNIT OF HEREDITY. • DNA is the molecule that is transmitted from parent to offspring that determines the physical outcome or plan. • DNA is instructions to make proteins. • PROTEINS are enzymes, hormones, and provide structure & color!!!!
GREGOR MENDEL: • The “father of genetics” • Austrian Monk • First to explain patterns of • inheritance. • Analyzed sweet pea plants • over 12 years. • Called the genes- “factors”
Mendel studied the inheritance patterns for physical traits like: 1.) flower petal color or 2) pea pod color or 3) pea shape. Table 14.1 The Results of Mendel’s F1 Crosses for Seven Characters in Pea Plants Purple or White flowers Ex. round or wrinkled peas
Mendel’s Experiments: First he created “pure breeding” strains of the plant through self-pollination. Parent Generation: • Pure for one of two contrasting traits • Result of many generations of self-pollination Genetic Cross #1 Mendel chose two plants pure for the two contrasting traits, (P1) generation, and crossed them (bred them together).
Mendel’s Experiments: (P1) Purple x (P1) White = First Filial Generation (F1): • Offspring of parent generation • All F1 plants looked like only one of the parents (in this case they all looked purple) • These were HYBRIDS Genetic Cross #2: Mendel then crossed two of the F1 plants (ewww! plant incest)
Mendel’s Experiments: Ppurple x Pwhite = F1purple F1purple x F1purple = Second Filial Generation (F2): • The traits from both parents reappear in these offspring • Some PURPLE • Some white Mendel found the ratio of expression to be 3:1.
IMPORTANT CONCLUSIONS: • If a trait exists in two contrasting forms one is dominant, one is recessive. The recessive one disappears in the F1 generation (ex. White). • Factors controlling traits occur in pairs. • The DOMINANT FACTOR prevents the recessive factor from being expressed. • Recessive factors are only expressed when both factors in the pair are recessive. • When pea plants reproduce, a factor pair is segregated (split) and each factor ends up in a separate gamete. Mendel’s law of segregation.
In modern terms: • Mendel’s factors are called genes. Some genes are dominant, others recessive. • Different forms of a single gene are called alleles. • Genes occur in pairs, at the same position on two chromosomes. • The gene position is called the locus. Alleles are alternate versions of a gene found on homologous Chromosomes.
5) The two chromosomes that contain the same genes are called: homologous. 6) If both alleles in a gene pair are the same, we say the pair is: homozygous (dominant or recessive) • If both alleles in a gene pair are different, we say the pair is: heterozygous. • When solving inheritance problems, use capital letters to represent dominant genes. • When solving inheritance problems, use lowercase letters to represent recessive genes. • A genotype is the combination of genes an individual possesses. • A phenotype is the appearance of an individual as the genes are being expressed.
The Punnett Square • Graphic tool used to solve inheritance problems • Predicts the possible gene combinations inherited by the offspring and their probability of occurring. • For example, a monohybrid cross: involves one character/trait. ex. Heterozygous X homozygous recessive (on board)
Segregation of alleles and fertilization as chance events If an individual is Heterozygous (Pp), flipping a coin- landing either heads or tails is similar to the probability of creating a gamete containing either the dominant or recessive gene in it… Both have a 50% probability or 1 out of 2 chances of occurring. Fertilization and creating of zygotes is the probability of the 2 gametes happening together. 1/2 x 1/2 or 1/4
A test-cross: is used to determine the genotype of an individual with a dominant phenotype (cross the individual in question with an individual with a recessive phenotype). ex. ____ X homozygous recessive ---> offspring
DIHYBRID CROSS • a two character/trait cross, with two gene pairs. • Results in 4 (not 2) phenotypes/looks. • Each parent can create a variety of gametes… how many? ex. AaBb X AaBb
There are 4 possible kinds of gametes: Mendel’s Law of Independent Assortment states when the two gene pairs are located on non-homologous chromosomes they segregate independently of each other. Aa Bb Results = 4 possible gamete combinations. (use FOIL) first: AB outer: Ab inner: aB last: ab (see board)
Arrogant, Beautiful X Arrogant, Beautiful Man Woman Both are Heterozygous for both genes… AaBb X AaBb What are their chances of having: arrogant, beautiful children? Modest, beautiful? arrogant, ugly? Etc.
AaBb X AaBb AB aB ab Ab AB Ab aB ab
AaBb X AaBb AB aB ab Ab AB Ab aB ab
AaBb X AaBb AB aB ab Ab AB Ab aB ab
Phenotype Ratio 9:3:3:1 9/16 Arrogant, Beautiful 3/16 Modest, Beautiful 3/16 Arrogant, Ugly 1/16 Modest, Ugly
Figure 14.7 Testing two hypotheses for segregation in a dihybrid cross
Now, it can’t be that easy can it? No it can’t. BESIDES THIS PATTERN OF INHERITANCE, called DOMINANCE or COMPLETE DOMINANCE, where having just ONE dominant allele results in the dominant phenotype, THERE ARE ALSO THE FOLLOWING PATTERNS: Incomplete dominance Codominance Multiple Alleles Polygenic Inheritance X linked inheritance
1)Incomplete Dominance: Both alleles are the same strength • Heterozygous individuals…express both alleles, The resulting phenotype is a mix (blend) of the two. • For example: Carnation flower color • The alleles are also written differently: AA’ or CR CW
2)Codominant Genes: • In a heterozygous individual both alleles affect the phenotype in separate distinguishable ways. • For example: Roan Horses have red and white hairs • Another example: A, B, and AB blood groups in humans.
3) Polygenic inheritance: When one phenotype is under the control of multiple gene pairs • For example: Human Skin Color • Alleles are… Dark and light • At least… three genes control the color of your skin • The phenotype is a result of… the cumulative effects of the dominant genes • Human skin color exists… as a gradient
4) Multiple Alleles: When more than two forms of the gene exist • For example: Human Blood Types (ABO blood groups) • Blood cells have a carbohydrate marker that is found on the surface of red blood cells. • These markers are called: A substance or B substance. • They are recognized by antibodies present in the blood serum of individuals for foreign carbohydrate markers. • There are two types of RBC antibody: Anti-A & Anti-B • Antibodies are a class of biological molecules called: Immunoglobulins • The genes for creating these antibodies are determined by the genes for creating the A or B substance.
Remember, the IA & IB alleles are CODOMINANT. • With human blood type, another marker called the Rh factor is denoted as + or -. • So, type O -, is the worst to be in terms of getting a blood transfusion… but is the best donor. • “universal donor”