Exploring Genetics: Mendel's Pea Plant Experiments
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Presentation Transcript
Genetics Ch. 11 pgs. 263-274
What is Genetics? • The study of heredity • Passing of traits from one generation to the next.
Gregor Mendel (1822) • Monk who worked in garden of monastery in Czech Republic. • Used pea plants to study heredity • Known as the “father of modern genetics”
Why Pea Plants? • Many contrasting traits • Reproduce sexually • Crosses can be controlled • Large # of offspring • Short lifecycles
A little more about pea plants… • Normally self pollinating • Pollen (sperm) fertilizes the ovule (egg) to produce offspring. • Produce genetically identical offspring. • Mendel used cross-pollination in his experiments. • Used pollen from one plant to fertilize another.
Genes and Dominance • Mendel studied 7 traits of pea plants • Traits are specific characteristics • For each trait there are 2 contrasting characteristics.
Generations • P- parental generation; original pair • F1- first filial generation; offspring from P generation • F2- second filial generation; offspring from F1
Mendel’s Findings • When he crossed P generation, F1 showed only one trait from the 2 parents. • Ex. Crossed Tall x Short short or tall
Mendel’s Conclusions • Biological inheritance is determined by factors passed from one generation to the next. • Factors are called genes. • 2 contrasting forms of the same trait are called alleles. • Ex. Trait is height, alleles are tall and short
Mendel’s Conclusions • Principal of dominance- Some alleles are dominant and some are recessive. • Dominant- when allele is present organism will always exhibit the dominant trait. • Recessive- only exhibit recessive trait if dominant allele is not present.
Alleles • Symbols for alleles: • A letter symbol is used for a specific trait. • Ex- height, Tt • Dominant- symbol is uppercase letter, T • Recessive- symbol is lowercase letter, t
Segregation • When Mendel crossed plants from the F1 generation traits from the P generation reappeared. P Tall x Short all tall plants F1 Tall x Tall some tall some short
Law of Segregation • The 2 alleles for each trait must separate when gametes form. • Parent only passes one allele for each trait to its offspring.
Probability and Punnett Squares • The principles of probability can be used to predict outcomes of genetic crosses. • Punnett square- diagram which shows possible Results of genetic crosses
Heterozygous vs. Homozgous • Hetero different • Homo same • Homozygous dominant- 2 identical dominant alleles (upper case). Expresses the dominant trait. • Ex- TT, tall
Heterozygous vs. Homozygous • Homozygous recessive- 2 identical recessive alleles (lower case). Expresses the recessive trait. • Ex- tt, short • Heterozygous- Have 2 different alleles for the same trait. ( upper and lower case) Express the dominant trait. • Ex. Tt, tall
Genotype vs Phenotype • Genotype- genetic make-up, letters • Tt vs tt • Phenotype- physical expression of the trait • Tall vs short
Sample Problem • In summer squash, white fruit color (W) is dominant over yellow fruit color (w). If a squash plant homozygous for white is crossed with a plant homozygous for yellow what will the phenotypic and genotypic ratios be?
Sample Problem • If 2 heterozygous white plants are crossed what will be the phenotypic and genotypic ratios?
Types of Genetic Crosses • Monohybrid one trait • Ex. Tt x tt • Dihybrid 2 traits • Ex- Hair and height • B= black, b=blonde, H=tall, h=short • What would be the phenotypes of a cross between the following parents? HhBb x HHbb
Types of genetic crosses cont. • Ex 2- Shape and color of pea plants R= round, r=wrinkled, Y= yellow, y=white What would be the phenotypes of a cross between a plant that is RRYY and a plant that is rrYy?
Genetics Review • If normal vision (N) is dominant to colorblindness (n) what are the chances that a heterozygous normal man and a colorblind woman will have a child with colorblindness?
Genetics Review • In some flowers red flowers (R) are dominant to white flowers (r) and tall stems (T) are dominant to short stems (t). What is the genotypic ratio for a cross between the following plants: RrTT x rrTT DD:DR:RD:RR
Types of genetic crosses cont. • Incomplete dominance • One allele is not completely dominant over the other • Heterozyous phenotype is somewhere between the 2 homozygous phenotypes. • 3 phenotypes instead of 2 • Use all capitol letters • Ex; RR= red, WW= white, RW= pink
Types of genetic crosses cont. • Inc. Dom. Example: In snapdragons the combination of red and white flowered plants can produce a pink flowered plant. RR= red WW= white RW= pink What would be the phenotypes of the offspring if a red flower were crossed with a pink flower?
Types of genetic crosses cont. • Co-dominance • Both alleles contribute to the phenotype • Ex- BB= black feather, WW= white feather, BW= black and white spotted feathers. • Same rules as inc. dom. but both phenotypes appear separately
Types of genetic crosses cont. • Co-dominance sample problem We have 2 fuzzy bunnies in class. One has black and white fur, the other is pure white. What are the genotypes of both rabbits? Black and White= White= What would be the phenotypes of the offspring if these rabbits mated?
Blood Type • Human blood type is determined by the type of protein found on the red blood cells (A or B). • Antigen- protein located on blood cell • Antibody- protein found in plasma, prevent foreign particles
Blood Type Example • What are the chances of a mother with A type blood and a father with O type blood having a child with A type blood?
Polygenic Inheritance • Some traits are polygenic- have more than one gene coding for the trait. • Ex- skin color, eye color, height
Polygenic Inheritance cont • 1. The weight of a fruit in a certain variety of squash is determined by two pairs of genes: AABB produces fruits weighing 4 lbs each and aabb produces fruits weighing 2 lbs each. A. How many pounds does each dominant allele add to the total weight of an individual squash?
Human Genetics • Chromosomes and Sex Determination • Humans have 23 pairs of chromosomes in autosomal cells. (diploid) • In sex cells there are only 23. (haploid) • Sex chromsomomes are the only pair that are non-homologous. • XX female • XY Male
Human Genetics • Ex- Cross between male and female for sex chromosomes: • Chance of boy: • Chance of girl:
Sex-Linked Inheritance • Some traits are linked to the X chromosome. • Most genetic disorders are X-linked. • Most X-linked disorders are expressed in males. WHY???
Sex- Linked Inheritance • Females usually act as carriers, they carry the recessive trait on one of their X chromosomes but the trait is not expressed.
Sex-Linked Inheritance • Ex- Normal colored vision (N) is dominant to colorblindness (n). Colorblindness is an X-linked trait. • Males are colorblind more then women and the gene is only found on the X chromosome. What is the chance that a man with normal vision and a woman who is a carrier will have a child that is colorblind?
Autosomal Traits • Genetic traits that are carried on the autosomal chromosomes.
Autosomal Recessive • Only expressed with homozygous recessive genotype. • Heterozygous genotype is carrier. • If trait present usually lethal disorder. • Ex- cystic fibrosis
Autosomal Recessive • Ex- C= no cystic fibrosis c= cystic fibrosis A female homozygous dominant for cystic fibrosis marries a man heterozygous for cystic fibrosis. What is the probability the child will have cystic fibrosis?
Autosomal Dominant • Expressed in homozygous dominant and heterozygous genotype. • No carrier (Either you have it or you don’t) • If trait is present usually lethal. • Ex- Huntington’s Disease
Autosomal Dominant • Ex H= Huntington’s disease h= normal A female who has Huntington’s disease and is heterozygous for the disorder marries a man who does not have the disorder. What is the probability that their child will have the disorder?
Pedigrees • Chart which shows the relationships within a family. • Shows the expression of genetic traits. • Symbols used: • Circle=female • Square=male • Half-shaded=carrier • Completely shaded=expresses trait
Pedigree • Used to determine if disorder is autosomal dominant, autosomal recessive or X-linked. • If only expressed in males and carried in female X-linked • If there are many carriers of both sexes autosomal recessive. • If no carriers and expressed in both sexes autosomal dominant
