GENETICS Mendels Laws of Heredity 1 Some Needed

GENETICS Mendel’s Laws of Heredity 1

Some Needed Vocabulary Genetics The study of Heredity Passing on of characteristics from parent to offspring Traits Characteristics that are inherited Ex: hair color; eye color; height; hairline shape Gene Parts of the DNA that code for traits Allele Different forms of a gene 2

Gregor Mendel A monk who studied heredity using garden pea plants Why garden peas? They reproduce sexually They have both male & female gametes on the same plant What did he do? He transferred pollen from one plant to another with different traits This is called “making a cross” Mendel first “crossed” tall plants with short 3

Mendel’s Monohybrid crosses Monohybrid cross Focuses on one (mono) trait 1 st generation Crossed a tall pea plant with a short pea plant All of the offspring were tall 2 nd generation Crossing the offspring of the 1 st generation ¾ of the offspring were tall; ¼ of the offspring were short 4

Mendel’s Findings Mendel found that each chromosome has two factors for each traits… called alleles Alleles = forms of genes Types of alleles Dominant The trait that has the ability to mask the other Represented with a capital letter (A) Recessive The trait that will be masked by the dominant trait… it will be there, but will not be 5

Homozygous vs. Heterozygous Homozygous (“homo” means “same”) Having two identical alleles for a trait TT, tt Heterozygous (“hetero” means “different”) Having two different alleles for a trait Tt 6

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Phenotype vs. Genotype Phenotype The way that an offspring looks; how the trait appears; it is ALWAYS a word. Tall, short Genotype The offspring’s genetic combination 8

Practice Identify whether it is “P” (a phenotype) or “G” (a genotype): Red Aa Smooth DD Bb Wrinkled Brown Ww 9

Mendel’s Rules 1. ) Principle of Dominance: Some alleles are dominant and some are recessive. 10

Mendel’s Rules: Principle of segregation The two alleles for each trait must separate when gametes are formed A parent will pass down AT RANDOM, only one allele for each trait 11

Mendel’s Rules Law of independent assortment Genes for different traits are inherited independently of one another 12

How do we determine what an offspring could look like? Can we ever be sure? 13

Punnett Squares are one way we can determine the probability that a particular offspring will be created from two parents. But first, what IS probability? 14

Punnett Squares When dealing with events that are random, probability represents the chance that a particular outcome will occur. What things are random in nature? Mating (in many species) Allele segregation during gamete formation 15

Punnett Squares A way to visually demonstrate probability in genetics is using the Punnett Square. 16

Mendel’s Punnett Square st 1 generation T = Tall t = short TT x tt T T t t Tt Tt 17

Interpreting the Punnett Square Tall = T Short = t T T t Tt Tt All offspring are heterozygous tall (Tt) Ratio 4: 0 (Tall : Short) Percentage 100% Tall Each offspring box represents 25% of the population of offspring produced 18

Mendel’s Punnett Square nd 2 generation o T Tall = ____ t Short = ___ Tt Tt Parents: _______ x _______ T t TT Tt Tt tt Homozygous Tall: TT o. Genotype: _______ 1 o. How many? ______ o. Heterozygous Tall: Tt o. Genotype: _______ 2 o. How many? ______ o. Short: tt o. Genotype: _______ 1 o. How many? ______ 3 o. TOTAL # OF TALL: _____ 1 o. TOTAL # OF SHORT: _____ 3: 1 o. Ratio of Tall : Short _____ o. Percentages: 75% o. Percent Tall: ______ 25% o. Percent Short: _____ 19

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Now, Let’s Practice. Work session: Grab a work sheet from the student resource center. Find Mrs. Foster and pick an egg. Go back to your seat and begin to work out your cross. REMEMBER: The eggs are for you to check your answer AFTER you have completed the Punnett square. 22

A little review: What is genetics? What is an allele? What does the Principle of Dominance say? What does the Principle of Independent Assortment state? 23

A little review: TT? tt? Tt? What is a phenotype? What is a genotype? 24

A little review What is the expected phenotypic ratio for the following cross? B=brown eyes b=blue Bb X Bb What is the genotypic ratio? 25

GENETICS Patterns of Heredity 26

Simple Mendelian Inheritance The type of inheritance we have discussed thus far (Mendel’s peas) Not all patterns in heredity are as simple… There is more than just dominant and recessive 27

Incomplete Dominance The phenotype of the heterozygote is intermediate (BLENDED) between those of the two homozygotes For Example: A cross between a particular Red flower and a White flower will produce a Pink flower 28

Punnett Square for Incomplete Dominance Cross: (RR) Red flower X (WW) White flower RR = Red WW = White RW = Pink R R W W RW RW 29

Codominance The phenotypes of both homozygotes is produced (SPOTTED) in the heterozygous individual For Example: Chick-fil-A cows! They have BOTH black AND white hairs. 30

Multiple Alleles Traits controlled by more than 2 alleles … For Example: Multiple alleles are commonly seen in the coat color of rabbits Also… Blood Typing… 31

Multiple Alleles – Blood Types Human blood types are determined by the presence or absence of certain molecules on the surfaces of red blood cells There are three alleles for blood type… IA………… written as “A” IB ………… written as “B” i ………… written as “O” 32

Blood Type Alleles Antigen Antibody Can give to Can get from 33

Multiple Alleles There are many possible allele combinations that can result in different blood types: IA IA and IA i = Type A blood IB IB and IBi = Type B blood Only one combination results Type AB and Type O blood: Type O blood : ii Type AB blood : IA IB 34

Punnet Squares with Multiple Alleles Set them up like usual: IA IA x I B IB A I B I A B I I 35

Sex-linked traits Traits that are controlled by genes located on sex chromosomes These traits help explain why some characteristics and disorders are more common in one sex than in the other Commonly referred to as “X-linked” because the trait is usually linked to the X-chromosome 36

Background Information Humans have 46 chromosomes; or 23 pairs of chromosomes Of these, 22 pairs are autosomes (self chromosomes) The last (23 rd) pair is different in males & females… these are the sex chromosomes Females have two “X” chromosomes (XX) Males have one “X” and one “Y” chromosome (XY) 37

“Sex-linked” traits Green-eyed male (Xb. Y) X Heterozygous Blue-eyed female (XBXb) B = Blue eyes b = Green eyes b X Y B X B b X X B X Y b X b b XX b XY 38

“Sex-linked” traits Green-eyed male (Xb. Y) X Heterozygous Blue-eyed female (XBXb) B = Blue eyes b = Green eyes b X Y X BX b X BY b X Xb. Y Xb. Xb OFFSPRING • MALES (XY) • Blue Eyes X BY • Genotype _______ • How many? _____1 • Green Eyes Xb. Y • Genotype _______ • How many? _____1 • FEMALES (XX) • Blue Eyes X BX b • Genotype _______ • How many? _____1 • Green Eyes Xb. Xb • Genotype _______ • How many? _____1 39

Sex-linked percentages Green-eyed male (Xb. Y) X Heterozygous Blue-eyed female (XBXb) B = Blue eyes b = Green eyes o. Percentages o. When figuring percents for sexlinked punnett squares, you must be careful to only count what is asked for… Ø% of females with blue eyes Xb Y B X X BX b X BY b X Xb. Xb Xb. Y Females Males 50% Ø% of males with blue eyes 50% Ø% of females with green eyes 50% Ø% of males with green eyes 40

Polygenic Inheritance When a trait is controlled by more than one gene For Example: Skin color is actually the accumulation of dominant genes for pigment in the skin 41