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 (ex: 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 (ex: a)

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

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

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

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

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 Now, draw the diagram on the board EXACTLY as it appears…

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

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

Punnett Squares 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?

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

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

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%

Now, Grab the WS from the table. Work session: Answer the questions. We will come back together in 20 minutes to review.

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?

GENETICS Patterns of Heredity

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

NON-MENDELIAN INHERITANCE So what’s next?? NON-MENDELIAN INHERITANCE

What does “non-Mendelian” mean? It means these inheritance patterns do not follow the rules Mendel discovered. So, remember how Mendel wanted to find “mediums” but never could? Well, mediums exist!!!

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 (RW) 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.

Codominance Punnett Square Same set up as Incomplete dominance. BB = Black WW = White BW = Chick-fil-A B B BW BW W BW BW W

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”

Blood Type Alleles Antigen Antibody Can give to Can get from

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

Practice: A man with type O blood marries a woman with type AB blood. What blood types could their children have?

Practice 2 A mother has type A blood. Her biological child has type O blood. What must the mother’s genotype be? Knowing the child’s blood type, what blood type(s) would not be possible for the child’s biological father to have?

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 Green-eyed female (XGXg) “Sex-linked” traits Blue-eyed male (XgY) X Heterozygous Green-eyed female (XGXg) G = Greeneyes g= Blue eyes Xg Y XG XGXg XGY Xg XgXg XgY

Heterozygous Green-eyed female (XBXb) “Sex-linked” traits OFFSPRING MALES (XY) Green Eyes Genotype _______ How many? _____ Blue Eyes FEMALES (XX) G = Green eyes g= blue eyes Blue-eyed male (XgY) X Heterozygous Green-eyed female (XBXb) Xg Y XG XGXg XgXg XgY XGY XGY 1 XgY 1 XGXg 1 XgXg 1

Sex-linked percentages G= Green eyes g = blue eyes Blue-eyed male (XgY) X Heterozygous Green eyed female (XGXg) Xg Y XG XGXg XgXg XgY XGY 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%

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