Mendelian Genetics. Introduction to the principles of Mendelian Genetics

Transcription

1 + Mendelian Genetics Introduction to the principles of Mendelian Genetics

2 + What is Genetics? n It is the study of patterns of inheritance and variations in organisms. n Genes control each trait of a living thing by controlling the formation of an organism's proteins. n Each cell contains two genes for each trait, one on the maternal chromosome and one on the paternal chromosome. n Remember: all cells (except gametes) are diploid, meaning they exist as a pair!

3 + Genes n The 2 genes may be of the same form or they may be of different forms. n These forms produce the different characteristics of each trait. n For example: A gene for plant height might occur in a tall form or a short form. n The different forms of a gene are called alleles. n The two alleles are segregated during the process of gamete formation (meiosis II) n Since organisms receive one gene for a chromosome pair from each parent, organisms can be heterozygous or homozygous for each trait.

4 + Who was Gregor Mendel? n Johann Mendel was born in 1822 in an area of Austria that is now part of the Czech Republic. n In 1843, he became a monk and took the name Gregor. While at the monastery, he was the caretaker of the garden. n In 1851, he went to the University of Vienna to study biology and math. n He is best known for his meticulous study of the inheritance of traits in pea plants.

5 + What did Mendel Study? n The popular theory of inheritance before Mendel came along was Blending, which stated that offspring are a mix of their parents traits (i.e. tall x short = medium) n Mendel s observations went against this theory. His pea plants were either identical to their parents, or completely different, not in-between. n He studied seven characteristics of pea plants: flower color & position, pod shape & color, stem length, and seed shape & color.

6

7 + Mendel s Methods n Mendel started his experiment with true-breeding pea plants n Plants that always produced offspring identical to themselves n Pea plants are self-pollinating, meaning the pollen from a flower can fertilize itself. n Mendel controlled the pollination of the plants by removing the anthers (male) from the flower. n Then, he carefully transferred pollen from other flowers on the stigma (female part) of the neutered flowers to cause cross-pollination.

8

9 Purple-flowered pea plant (dominant) White-flowered pea plant (recessive)

10 + Mendel s First Experiment n Mendel called the true-breeding parent plants the P generation. He crossed true-breeding purple flowered pea plants with true-breeding white flowered plants. n All of the offspring had purple flowers! He called these offspring the F1 generation (for first filial). These plants were hybrids. n When he let the F1 offspring self-pollinate, about 75% of the offspring had purple flowers, but 25% had white flowers. He called these offspring the F2 generation.

11 P generation X white purple F1 generation purple purple purple purple F2 generation white purple purple purple

12 + Mendel s Results & Analysis n Mendel proposed that there must be a heritable factor that was passed from parents to offspring. n Today we call that heritable factor a gene n Mendel wanted to know why the white flowered plants disappeared in the F1 generation, but then reappeared in the F2 generation. n He also wondered why he always observed a 3:1 ratio in the F2 generation of purple:white flowers. n Mendel carried out identical experiments for pod shape & color; seed shape & color; always observing the same results and ratios.

13 + Mendel s Law of Dominance n Law states that there are different versions of genes, called alleles, that account for the variations in traits. n States that some alleles are dominant whereas others are recessive n An organism with a dominant allele for a particular trait will always have that trait expressed in the organisms. n An organisms with a recessive allele for a particular trait will only have that trait expressed when the dominant allele is not present.

14 + Homozygous n When an organism has two identical alleles for a particular trait that organisms is said to be homozygous for that trait n The paternal chromosome and the maternal chromosome have the same form of the gene. n They are either both dominant or both recessive n Examples: (For blue color, B = blue and b = pink) n BB n bb

15 + Heterozygous n When an organism has two different alleles for a particular trait that organism is said to be heterozygous for that trait n The paternal chromosome and the maternal chromosome have different forms of the gene; one is dominant and one is recessive n Example: (color, B = blue and b=pink) n Bb (blue)

16 + Genotype n Genotype: n The genetic make-up of an organism reveals the type of alleles that an organisms has inherited for a particular trait. n The genotype for a particular trait is usually represented by a letter. n The capital letter representing the dominant gene. n The lower-case letter representing the recessive gene. n Examples: n TT represents a homozygous dominant genotype n tt represents a homozygous recessive genotype n Tt represents a heterozygous genotype

17 + Phenotype n Phenotype: n The physical characteristics of an organism is a description of the way that a trait is expressed in the organism n Organism with the genotype of BB or Bb would have a phenotype of black. n Organism with the genotype of bb would have a phenotype of white.

18 + Law of Segregation n The law of segregation explains how alleles are separated during meiosis n Each gamete receives one of the two alleles that the parent carries for each trait. n Each gamete has the same chance of receiving either one of the alleles for each trait. n During fertilization (when the egg and sperm unite), each parent organism donates one copy of each gene to the offspring.

19 + Law of Segregation

20 + Law of Independent Assortment n The law of independent assortment states that the segregation of the alleles of one trait does not affect the segregation of the alleles of another trait n Genes on separate chromosomes separate independently during meiosis n This law holds true for all genes unless the genes are linked. n In this case, the genes that do not independently segregate during gamete formation, usually because they are in close proximity on the same chromosome.

21 + Punnett Squares n The principles of Mendelian genetics can be used to predict the inherited traits of the offspring. n A punnett square can be used to predict the probable genetic combinations in the offspring that result from different parental allele combinations that are independently assorted.

22 + Punnett Squares n A monohybrid cross examines the inheritance of one trait. The cross could be any of the following: n homozygous-homozygous n heterozygous heterozygous n Heterozygous - homozygous

23 + Punnett Squares n Example: n Represent the probable outcome of two heterozygous parents with the trait for height: T = dominant (tall) and t = recessive (short) n Tt x Tt n The parents are the F1 generation and the offspring are the F2 generation n The square shows the following possible genotypes: n 1:4 ratio (25%) for two dominant alleles n 1:4 ration (25%) for two recessive alleles n 2:4 or 1:2 ratio (50%) for one dominant and one recessive allele n The square shows the following phenotypes are possible: n 3:4 ratio (75%) to express the tall trait n 1:4 ratio (25%) to express the short trait

24 + Punnett Square n Remember that only one of the options is possible for the offspring n n n Not all 4 options are made into one offspring A punnett square just gives you all the potential outcomes for the offspring Practice problem: n What are the potential genotypic and phenotypic outcomes if two heterozygous parents for body color are crossed? n Male parent = blue n Female parent = red n Blue is dominant over red X

25 + Punnett Square n A dihybrid cross examines the inheritance of two different traits n Example: n Homozygous parents for shape and color are crossed n R = dominant round; r = recessive wrinkled; Y = dominant yellow; y = recessive green n rryy x RRYY n The parents are the F 1 generation and the offspring are the F 2 generation

26 + Punnett Square n Dihybrid Example Continued n All of the offspring for this generation would predictably have the same genotype, heterozygous for both traits (RrYy) n All of the offspring for this generation would predictably have the same phenotype, round and yellow (16/16 will be round and yellow