MENDELIAN GENETICS Introduction to Genetics and heredity Gregor
- Slides: 75
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 F 1 generation – first filial generation (offspring of P) F 2 generation – second filial generation (offspring of F 1; 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.
• 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 What is the phenotype? Is it heterozygous or homozygous? Homozygous dominant or recessive?
If a pea plant is Tt What is the phenotype? Is it heterozygous or homozygous? Homozygous dominant or recessive?
If a pea plant is tt What is the phenotype? Is it heterozygous or 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 F 1 generation – first filial generation (offspring of P) F 2 generation – second filial generation (offspring of F 1; 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 TT tt
Monohybrid cross – F 1 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 F 1 generation is the result of crosses between individuals of the P generation…this is called crosspollination • All of the F 1 offspring grew to be as tall as the tall parent.
Monohybrid cross – F 2 generation • Mendel allowed the tall plants in this F 1 to selfpollinate. • After the seeds formed, he planted them and counted more than 1000 plants in this F 2. • ¾ of the plants were as tall as the tall plants in the P and F 1 generations; ¼ of the plants were short
• Only 1 trait is seen in the F 1, but then 2 traits are seen in the F 2? • 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 F 1 generation, then reappear in ¼ of the F 2 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 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 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 TT tt t t Tt Tt T Tt Genotypes: 100% T t Tt Phenotypes: 100% Tall plants
Monohybrid cross: F 2 generation • If you let the F 1 generation self-fertilize, the next monohybrid cross would be: Tt (tall) T t TT Tt Tt tt (tall) Genotypes: 1 TT= Tall 2 Tt = Tall 1 tt = dwarf Genotypic ratio= 1 TT: 2 Tt: 1 tt 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 Watch this video for more help with monohybrid crosses T T t t Tt Tt
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): PP Pp pp ALL PURPLE (Pp)
Cross the P generation: PP P P pp p p Pp Pp Genotypes: 4 Pp Phenotypes: 4 Purple
Cross the F 1 generation: Pp P p PP Pp Pp pp Genotypes: 1 PP 2 Pp 1 pp Phenotypes: 3 Purple 1 White
1. Principle of Dominance: One allele masks another; one allele is dominant over the other in the F 1 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.
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’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 (12 M) • 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 = normal C c = carrier, no symptoms c c = has cystic fibrosis c C CC Cc cc
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 • If you get 50% white, 50% purple flowers, then the unknown parent was Pp… p p p Pp Pp P p Pp pp
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 v 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 v In pea plants, purple flowers (P) are dominant over white flowers (p). Show the cross between two heterozygous plants. GENOTYPES: - PP (25%) Pp (50%) pp (25%) - ratio 1 PP: 2 Pp: 1 pp PHENOTYPES: - purple (75%) white (25%) - ratio 3 Purple: 1 White P p P PP Pp pp
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 1. 2. 3. 4. 5. Incomplete Dominance Codominance Multiple Alleles Polygenic Traits Sex-Linked Traits
1. Incomplete Dominance v A third (new) phenotype appears in the heterozygous condition as a BLEND of the dominant and recessive phenotypes. v Ex - Dominant Red (R) + Recessive White (r) = Hybrid Pink (Rr) RR = red rr = white Rr = pink
Example: Incomplete Dominance v Show the cross between a pink and a white flower. GENOTYPES: - RR (0%) Rr (50%) rr (50%) - ratio 1: 1 R r r Rr rr PHENOTYPES: - pink (50%); white (50%) - ratio 1: 1
PRACTICE: Incomplete Dominance v Show the cross between a red and a white flower… GENOTYPES: - RR Rr rr - ratio PHENOTYPES: - Red: White: - ratio Pink:
PRACTICE: Incomplete Dominance v Show the cross between two offspring of the previous cross… GENOTYPES: - RR Rr rr - ratio PHENOTYPES: - Red: White: - ratio Pink:
2. Codominance v In the heterozygous condition, both alleles are expressed equally, but NO blending! Represented by using two DIFFERENT capital letters. v Example: Dominant Black (B) + Dominant White (W) = Speckled Black and White Phenotype (BW) v Sickle Cell Anemia - NN = normal cells SS = sickle cells NS = some of each
Codominance Example: Speckled Chickens v. BB = black feathers v. WW = white feathers v. BW = black & white speckled feathers v. Notice – NO GRAY! NO BLEND! Each feather is either black or white
Codominance Example: Rhodedendron v R = allele for red flowers v W = allele for white flowers v Cross a homozygous red flower with a homozygous white flower.
Codominance Example: Roan cattle vcattle can be red (RR – all red hairs) white (WW – all white hairs) roan (RW – red and white hairs together)
Codominance Example: Appaloosa horses v Gray horses (GG) are codominant to white horses (WW). The heterozygous horse (GW) is an Appaloosa (a white horse with gray spots). v Cross a white horse with an appaloosa horse. W W G GW GW W WW WW
PRACTICE: Codominance v. 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: 1 SS S NS SS PHENOTYPES: - Normal (0%) Carrier (50%) - ratio 1 carrier: 1 sick Sick (50%)
PRACTICE: Codominance v. Show the cross between two carrier parents… GENOTYPES: -NN NS SS - ratio PHENOTYPES: - Normal - ratio Carrier Sick
3. Multiple Alleles v. 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 v. 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 Can (antigen) Donate Receive Possible on RBC Blood Phenotype Genotype(s) surface To From A I Ai I AI A A A, AB A, O B IB i IB IB B B, AB B, O AB AB A, B, AB, O O AB O I AI B ii
Problem: Multiple Alleles v. Show the cross between a mother who has type O blood and a father who has type AB blood. GENOTYPES: - IA (50%) IB (50%) - ratio 1 IAi : 1 IBi PHENOTYPES: - type A (50%) type B (50%) - ratio 1 type A : 1 type B i i IA I Ai IB IB i
PRACTICE: Multiple Alleles v 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 vtraits produced by multiple genes vexample: 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.
r e b m u n ? e t e a s h u W yo o d
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 - Color. Blindness Red-Green Colorblindness is a sex-linked recessive disorder. Cross a colorblind female (n) with a normal-vision male (N). n. X n x X NY X GENOTYPES: XNXN: ___ XNXn: ___ Xn. Xn: ___ XNY: ___ Xn. Y: ___ Ratio: PHENOTYPES: Normal Male___ Normal Female ___ Colorblind Male ___ Colorblind Female ___ Ratio:
Sex-Linked Traits - Hemophilia • The gene for hemophilia • Males are more likely to is found on the X get hemophilia. chromosome • Females have the • It is a recessive disorder. possibility of being heterozygous for • (referred to as a sexhemophilia. (This makes linked recessive them a carrier) disorder)
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. XH Xh x XH Y GENOTYPES: XHXH: ___ XHXh: ___ Xh. Xh: ___ XHY: ___ Xh. Y: ___ 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: ___ Xh. Xh: ___ XHY: ___ Xh. Y: ___ 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.
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