Mendel’s Laws of Heredity GENETICS Mendel’s Laws of Heredity

Some Needed Vocabulary Genetics The study of Heredity 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

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 plants

Mendel’s Monohybrid crosses Focuses on one (mono) trait 1st generation Crossed a tall pea plant with a short pea plant All of the offspring were tall 2nd generation Crossing the offspring of the 1st generation ¾ of the offspring were tall; ¼ of the offspring were short 3:1 ratio (tall to short)

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 expressed Represented with a lowercase letter (a)

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

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

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

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

Punnett square Short way to find the expected proportions of possible genotypes in the offspring of a testcross

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

Interpreting the Punnett Square 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 Tall = T Short = t T t

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

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?

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

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?

Patterns of Heredity “Beyond Mendel” GENETICS Patterns of Heredity “Beyond Mendel”

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

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

Punnett Square for Incomplete Dominance Cross: (RR) Red flower X (WW) White flower 100% PINK FLOWERS (RR') RR = Red WW = White RW = Pink R R W R W R W W R W R W

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.

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

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”

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

Punnet Squares with Multiple Alleles Set them up like usual: IA IA x IB IB IA IA IAIB IB IAIB IB IAIB IAIB

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

Background Information Humans have 46 chromosomes; or 23 pairs of chromosomes Of these, 22 pairs are autosomes (self chromosomes) The last (23rd) 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)

Heterozygous Blue-eyed female (XBXb) “Sex-linked” traits Green-eyed male (XbY) X Heterozygous Blue-eyed female (XBXb) B = Blue eyes b = Green eyes Xb Y XB XBXb XBY Xb XbXb XbY

Heterozygous Blue-eyed female “Sex-linked” traits OFFSPRING MALES (XY) Blue Eyes Genotype _______ How many? _____ Green Eyes FEMALES (XX) B = Blue eyes b = Green eyes Green-eyed male X Heterozygous Blue-eyed female Xb Y XB XBXb XbXb XbY XBY XBY 1 XbY 1 XBXb 1 XbXb 1

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