MENDELIAN GENETICS Introduction to Genetics and heredity Gregor Mendel – a brief bio Genetic terminology (glossary) Monohybrid crosses Patterns of inheritance Test cross Beyond Mendelian Genetics – incomplete dominance & codominance

Introduction to Genetics GENETICS – branch of biology that deals with heredity and variation of organisms. Chromosomes carry the hereditary information (genes) Condensed DNA (tightly-packed chromatin) DNA  RNA  Proteins

Chromosomes (and genes) occur in pairs… Homologous Chromosomes New combinations of genes occur in sexual reproduction…WHY? Fertilization from two parents Crossing over in Meiosis I

Gregor Mendel – Austrian monk who was the first person to study the passing on of characteristics from parents to offspring. He did this by crossing male and female pea plants and observing the results. P generation – original parents F1 generation – first filial generation (offspring of P) F2 generation – second filial generation (offspring of F1; grandchildren of P)

Mendel’s peas Mendel looked at seven traits or characteristics of pea plants:

In 1866 he published Experiments in Plant Hybridization, (Versuche über Pflanzen-Hybriden) in which he established his three Principles of Inheritance He tried to repeat his work in another plant, but didn’t work because the plant reproduced asexually! Work was largely ignored for many years, until ~1900, when 3 independent botanists rediscovered Mendel’s work.

Mendel was the first biologist to use Mathematics to explain his results quantitatively. Mendel predicted: - The concept of genes - That genes occur in pairs - That one gene of each pair is present in the gametes

Genetics terms you need to know: Trait – a characteristic that is inherited Examples: Hair color, eye color Gene – a segment of DNA containing instructions for a trait (which codes for a protein) Genome – the entire set of genes in an organism

Genetics terms you need to know: Allele – One version of a gene that codes for a trait. An individual always has two alleles for each trait… these occupy the same position on homologous chromosomes (like ‘flavors’ of a trait) Where do the alleles come from? One from mom, one from dad Alleles are shown with letter combinations (TT or Tt or tt) Locus – a fixed location on a strand of DNA where a gene or one of its alleles is located.

Recessive – an allele that is “masked” or hidden by a dominant allele. Homozygous – having identical genes (one from each parent) for a particular characteristic (known as “pure”) Either TT or tt Heterozygous – having two different genes for a particular characteristic (also known as a “carrier”… doesn’t express the recessive, but can pass it along) – Tt Dominant – the allele of a gene that “masks” or covers the expression of recessive allele. Expressed with a CAPITAL letter (T) Recessive – an allele that is “masked” or hidden by a dominant allele. Expressed with a lowercase letter (t)

Genotype – the genetic makeup of an organism Phenotype – the physical appearance of an organism (determined by both an organism’s genotype + environment) What is the relationship between genotype and phenotype?

If a pea plant is TT  tall What is the phenotype? Is it heterozygous or homozygous? homozygous Homozygous dominant or recessive?

If a pea plant is Tt  tall What is the phenotype? Is it heterozygous or homozygous? heterozygous Homozygous dominant or recessive?

If a pea plant is tt  short What is the phenotype? Is it heterozygous or homozygous? homozygous Homozygous dominant or recessive?

Cross-pollination – when we choose which plants to cross to obtain the next generation Self-pollination – when the plants are allowed to reproduce by pollinating naturally

Monohybrid cross: a genetic cross involving a single pair of genes (one trait); parents differ by a single trait. P generation – original parents F1 generation – first filial generation (offspring of P) F2 generation – second filial generation (offspring of F1; grandchildren of P)

Monohybrid cross Crossing two pea plants that differ in stem size, one tall one short T = allele for Tall t = allele for short/dwarf TT = homozygous tall plant tt = homozygous dwarf plant T T  t t

Monohybrid cross – F1 generation Mendel selected a six-foot-tall pea plant that came from a population of pea plants, all of which were over six feet tall. The F1 generation is the result of crosses between individuals of the P generation…this is called cross-pollination All of the F1 offspring grew to be as tall as the tall parent.

Monohybrid cross – F2 generation Mendel allowed the tall plants in this F1 to self-pollinate. After the seeds formed, he planted them and counted more than 1000 plants in this F2. ¾ of the plants were as tall as the tall plants in the P and F1 generations; ¼ of the plants were short

Only 1 trait is seen in the F1, but then 2 traits are seen in the F2? Mendel’s answer: dominant and recessive “factors” (we now call them alleles) In every cross between homozygous dominant x homozygous recessive individuals, he found that the recessive trait seemed to disappear in the F1 generation, then reappear in ¼ of the F2 plants

Punnett square We use the Punnett square to predict the genotypes and phenotypes of the offspring. Monohybrid cross – cross involving only one characteristic Dihybrid cross – cross involving two different characteristics

The two alleles from one parent are listed on top of the square The two alleles from the other parent are listed on the left side.

Using a Punnett Square TT and t t STEPS: 1. determine the genotypes of the parent organisms 2. write down your "cross" (mating) 3. draw a p-square Parent genotypes: TT and t t

Punnett square T T t T T  t t T t STEPS: 4. "split" the letters of the genotype for each parent & put them "outside" the p-square 5. determine the possible genotypes of the offspring by filling in the p-square (drop and slide method) 6. summarize results (genotypes & phenotypes of offspring) T T T T  t t T t t Genotypes: 100% T t Phenotypes: 100% Tall plants

Monohybrid cross: F2 generation If you let the F1 generation self-fertilize, the next monohybrid cross would be: T t  T t (tall) (tall) Genotypes: 1 TT= Tall 2 Tt = Tall 1 tt = dwarf Genotypic ratio= 1TT:2Tt:1tt T t T T T t t t T t Phenotype: 3 Tall 1 dwarf Phenotypic ratio= 3 tall:1 short

Secret of the Punnett Square Key to the Punnett Square: Determine the gametes of each parent… How? By “splitting” the genotypes of each parent: If this is your cross T T  t t The gametes are: T T t t

Once you have the gametes…  t t t t T t Watch this video for more help with monohybrid crosses T T

Another example: Flower color For example, flower color: P = purple (dominant) p = white (recessive) If you cross a homozygous Purple (PP) with a homozygous white (pp):  P P p p ALL PURPLE (Pp) P p

Cross the P generation: P P pp  p p Genotypes: 4 Pp P p P Phenotypes: 4 Purple P

Cross the F1 generation: P p P p  Genotypes: PP Pp 1 pp P p P P P p p p P Phenotypes: 3 Purple 1 White p

Principles of heredity developed by Mendel 1. Principle of Dominance: One allele masks another; one allele is dominant over the other in the F1 generation 2. Principle of Segregation: When gametes are formed, the pairs of hereditary factors (genes) become separated, so that each sex cell receives only one allele (B or b) to each offspring.

Principles of heredity developed by Mendel 3. Principle of Independent Assortment: “Members of one gene pair segregate independently from other gene pairs during gamete formation” Genes get shuffled – these many combinations are one of the advantages of sexual reproduction

Relation of gene segregation to meiosis… There’s a correlation between the movement of chromosomes in meiosis and the segregation of alleles that occurs in meiosis

Mendel & today… Mendel’s work has held true to this day Genes on chromosomes control traits Allele - the coding for particular traits We now use letters to show these dominant and recessive alleles One change = independent assortment only applies to genes that are far apart on a chromosome or on different chromosomes

Human case: CF Mendel’s Principles of Heredity apply universally to all organisms. Cystic Fibrosis: a lethal genetic disease affecting Caucasians. Caused by mutant recessive gene carried by 1 in 20 people of European descent (12M) One in 400 Caucasian couples will be both carriers of CF – 1 in 4 children will have it. CF disease affects transport in tissues – mucus is accumulated in lungs, causing infections.

Inheritance pattern of CF IF two parents carry the recessive gene of Cystic Fibrosis (c), that is, they are heterozygous (Cc), one in four of their children is expected to be homozygous for cf and have the disease: C c C C C c c c C c C C = normal C c = carrier, no symptoms c c = has cystic fibrosis

Probabilities… Of course, the 1 in 4 probability of getting the disease is just an expectation, and in reality, any two carriers may have normal children. However, the greatest probability is for 1 in 4 children to be affected. What is the % chance? Important factor when prospective parents are concerned about their chances of having affected children. Now, 1 in 29 Americans is a symptom-less carrier (Cf cf) of the gene.

Test cross When you have an individual with an unknown genotype, you do a test cross. Test cross: Cross the unknown genotype individual with a homozygous recessive individual. For example, a plant with purple flowers can either be PP or Pp… therefore, you cross the plant with a pp (white flowers, homozygous recessive) P ?  pp

Test cross If you get all 100% purple flowers, then the unknown parent was PP… P P P p p If you get 50% white, 50% purple flowers, then the unknown parent was Pp… P p P p p p p

Test cross practice… B – black fur b – white fur I have a black rabbit but I don’t know whether it is BB or Bb. How can I check?

Mendelian Genetics: Dominant & Recessive Review One allele is DOMINANT over the other (because the dominant allele can “mask” the recessive allele) genotype: PP genotype: pp genotype: Pp phenotype: purple phenotype: white phenotype: purple

Review Problem: Dominant & Recessive In pea plants, purple flowers (P) are dominant over white flowers (p). Show the cross between two heterozygous plants. P p GENOTYPES: - PP (25%) Pp (50%) pp (25%) - ratio 1 PP:2 Pp:1 pp P p PP Pp Pp pp PHENOTYPES: - purple (75%) white (25%) - ratio 3 Purple:1 White

NON- MENDELIAN GENETICS Mendel was lucky! Traits he chose in the pea plants showed up very clearly… The pattern of inheritance is usually normal, or dominant/recessive. In these cases, phenotypes are easy to recognize. But sometimes phenotypes are not very obvious. Let’s look at a few other patterns of inheritance…

NON- MENDELIAN GENETICS Incomplete Dominance Codominance Multiple Alleles Polygenic Traits Sex-Linked Traits

1. Incomplete Dominance A third (new) phenotype appears in the heterozygous condition as a BLEND of the dominant and recessive phenotypes. Ex - Dominant Red (R) + Recessive White (r) = Hybrid Pink (Rr) RR = red rr = white Rr = pink

Example: Incomplete Dominance Show the cross between a pink and a white flower. GENOTYPES: R r - RR (0%) Rr (50%) rr (50%) - ratio 1:1 r Rr rr Rr rr PHENOTYPES: - pink (50%); white (50%) - ratio 1:1

PRACTICE: Incomplete Dominance Show the cross between a red and a white flower… GENOTYPES: - RR Rr rr - ratio PHENOTYPES: - Red: White: Pink: - ratio

PRACTICE: Incomplete Dominance Show the cross between two offspring of the previous cross… GENOTYPES: - RR Rr rr - ratio PHENOTYPES: - Red: White: Pink: - ratio

2. Codominance NS = some of each NN = normal cells SS = sickle cells In the heterozygous condition, both alleles are expressed equally, but NO blending! Represented by using two DIFFERENT capital letters. Example: Dominant Black (B) + Dominant White (W) = Speckled Black and White Phenotype (BW) Sickle Cell Anemia - NS = some of each NN = normal cells SS = sickle cells sick

Codominance Example: Speckled Chickens BB = black feathers WW = white feathers BW = black & white speckled feathers Notice – NO GRAY! NO BLEND! Each feather is either black or white

Codominance Example: Rhodedendron R = allele for red flowers W = allele for white flowers Cross a homozygous red flower with a homozygous white flower.

Codominance Example: Roan cattle cattle can be red (RR – all red hairs) white (WW – all white hairs) roan (RW – red and white hairs together)

Codominance Example: Appaloosa horses Gray horses (GG) are codominant to white horses (WW). The heterozygous horse (GW) is an Appaloosa (a white horse with gray spots). Cross a white horse with an appaloosa horse. W W G W GW GW WW WW

PRACTICE: Codominance Show the cross between an individual with sickle-cell anemia (SS) and another who is a carrier (NS) but not sick… N S GENOTYPES: -NN (0%) NS (50%) SS (50%) - ratio 1 NS:1SS S NS SS PHENOTYPES: - Normal (0%) Carrier (50%) Sick (50%) - ratio 1 carrier:1sick

PRACTICE: Codominance Show the cross between two carrier parents… GENOTYPES: -NN NS SS - ratio PHENOTYPES: - Normal Carrier Sick - ratio

3. Multiple Alleles There are more than two alleles for a gene. Ex – blood type consists of two dominant and one recessive allele options. Allele A and B are dominant over Allele O.

Multiple Alleles: Rabbit Fur Colors Fur colors (determined by 4 alleles): full, chinchilla, himalayan, albino

Multiple Alleles: Blood Types (A, B, AB, O) Rules for Blood Types: A and B are co-dominant (Both show) AA or IAIA = type A BB or IBIB = type B AB or IAIB = type AB A and B are dominant over O (Regular dom/rec) AO or IAi = type A BO or IBi = type B OO or ii = type O

Allele (antigen) on RBC surface Phenotype Possible Genotype(s) Allele (antigen) on RBC surface Can Donate Blood To Can Receive Blood From A IAi IAIA A, AB A, O B IBi IBIB B, AB B, O AB IAIB A, B, AB, O O ii

Problem: Multiple Alleles Show the cross between a mother who has type O blood and a father who has type AB blood. GENOTYPES: i i - IA (50%) IB (50%) - ratio 1 IAi : 1 IBi IA IB IAi IBi PHENOTYPES: - type A (50%) type B (50%) - ratio 1 type A : 1 type B

PRACTICE: Multiple Alleles Show the cross between a mother who is heterozygous for type B blood and a father who is heterozygous for type A blood. GENOTYPES: IAIB (__%); IBi (__%); IAi (__%); ii (__%) - ratio PHENOTYPES: type AB (__%); type B (__%) type A (__%); type O (__%) - ratio

4. Polygenic Traits traits produced by multiple genes example: skin color

Chromosome Review… Most species of animals and plants carry a pair of chromosomes that determine the individuals sex. These are called sex chromosomes. All other chromosomes are called autosomal

The Y chromosome The Y chromosome is much smaller than the X. It carries a small number of genes, most of which are for “male characteristics”

X chromosome All human eggs contain the X chromosome. The X chromosome contains genes that code for all aspects of femaleness, as well as genes unrelated to gender. Including genes for: Vision Immunity

Which parent determines the sex of an offspring? DAD Why? All moms have the genotype XX. When egg cells are made, they will all carry a single X chromosome. All dads have the genotype XY. When sperm cells are made, 50% will have an X chromosome and 50% will have a Y chromosome. Therefore, males and females are born in roughly a 50:50 ratio.

5. Sex-Linked Traits Those traits that are controlled by genes on the X or Y chromosomes. NOTE: The Y chromosome is much smaller than the X chromosome and only contains a few genes. Disorders caused by sex-linked traits are more common in boys. WHY? Most sex-linked traits are on the X chromosome. example: red-green colorblindness

Sex-Linked Traits - recessive In males, there is no second X chromosome to “mask” a recessive gene. If they get an X with the disorder, they have it. Girls must inherit defective X’s from both parents to have the disorder.

What number do you see?

Sex-Linked Traits - Colorblindness A: 29, B: 45, C: --, D: 26  Normal vision ----------------------------------------- A: 70, B: --, C: 5, D: --  Red-green color blind A: 70, B: --, C: 5, D: 6  Red color blind A: 70, B: --, C: 5, D: 2  Green color blind

Sex-Linked Traits - ColorBlindness Red-Green Colorblindness is a sex-linked recessive disorder. Cross a colorblind female (n) with a normal-vision male (N). XnXn x XNY GENOTYPES: XNXN: ___ XNXn: ___ XnXn: ___ XNY: ___ XnY: ___ Ratio: PHENOTYPES: Normal Male___ Normal Female ___ Colorblind Male ___ Colorblind Female ___ Ratio:

Sex-Linked Traits - Hemophilia The gene for hemophilia is found on the X chromosome It is a recessive disorder. (referred to as a sex-linked recessive disorder) Males are more likely to get hemophilia. Females have the possibility of being heterozygous for hemophilia. (This makes them a carrier)

Sex-Linked Traits - Hemophilia Having hemophilia is recessive (Xh) to being normal (XH). The heterozygous female is called a carrier. Cross a carrier female with a normal male. XHXh x XHY GENOTYPES: XHXH: ___ XHXh: ___ XhXh: ___ XHY: ___ XhY: ___ Ratio: PHENOTYPES: Normal Male___ Normal Female ___ Hemophiliac Male ___ Hemophiliac Female ___ Ratio:

PRACTICE - Hemophilia Cross a carrier female with a male hemophiliac. _____ x _____ GENOTYPES: XHXH: ___ XHXh: ___ XhXh: ___ XHY: ___ XhY: ___ Ratio: PHENOTYPES: Normal Male___ Normal Female ___ Hemophiliac Male ___ Hemophiliac Female ___ Ratio:

Question… Why are sex-linked traits more common in males than in females? Because a male only has to inherit ONE recessive allele in order to get a sex-linked trait and a female has to inherit TWO recessive alleles in order to acquire the sex-linked trait.