Presentation on theme: "Mendelian genetics. *Gregor Mendel is considered the “father of genetics.” *In 1843, at the age of 21, he became a monk in Austria where he was put in."— Presentation transcript:
Mendelian genetics. *Gregor Mendel is considered the “father of genetics.” *In 1843, at the age of 21, he became a monk in Austria where he was put in charge of tending the monastery garden. *In 1851, he was sent to the University of Vienna where he studied science, math, and statistics.
*Gregor finished college and returned back to the monastery as a very well-educated monk. *He began working in the garden again and noticed several things….. *Many of the tall plants he grew produced tall offspring, while some produced short offspring. *Some of the plants with yellow seeds had offspring with yellow seeds, while others had green seeds. *Mendel believed that plants inherited their traits from their parents. He decided to see if he could find a predictable pattern to the way pea plants inherited traits.
*Mendel developed pure strains of plants by self-pollinating plants with the trait he wanted. *The pure strains each had a contrasting trait. *He called these pure strains the P1 (parental ) generation.
Plants reproduce when pollen from the anther is transferred to the stigma and the ovules are fertilized.
*Mendel developed 14 pure plant strains. Each characteristic had two contrasting traits. *Mendel cross pollinated the pure strains and called the offspring the F1 (1 st filial) generation.
*The F1 plants were then self-pollinated, and Mendel called their offspring the F2 (2 nd filial) generation. *Think of this as the parents, kids, & grandkids. *Gregor Mendel made thousands of crosses and found a predictable pattern. *In the F1 generation, the recessive trait was “hidden.” *In the F2 generation, the recessive trait re- appeared in a 3:1 ratio with the dominant trait. *Mendel concluded that there are three principles that control heredity.
Mendel’s Principles of Heredity. Principle of Dominance – when 2 forms of the same gene are present the dominant allele is expressed. Principle of Segregation – in meiosis two alleles separate so that each gamete (sex cell) only receives one form of the gene. You get a trait either from your mother or father, not both. Principle of Independent Assortment – each trait is inherited independently from other traits. Height and hair color are not connected.
Genes. *Mendel’s factors are now called genes. *A gene is a segment of DNA on a chromosome that controls a hereditary trait. *Chromosomes occur in pairs. *One chromosome comes from each parent. *The gene segment on each chromosome is called an allele.
*Dominant alleles are represented by a capital letters (T, R). *Recessive alleles are represented by lower case letters (t,r). *A homozygous dominant genotype (TT, RR) always shows the dominant trait. *A heterozygous dominant genotype (Tt, Rr) always shows the dominant trait, but is a carrier of the recessive trait. *A homozygous recessive genotype (tt, rr) always shows the recessive trait.
*The genotype is the genetic makeup of the Individual and is expressed in letters (TT, Tt, tt). *The phenotype is what the individual actually looks like (tall, short). Genetic crosses. A monohybrid cross is a cross between individuals that involves only one pair of contrasting traits. (TT x Tt)
1. Cross a plant that is homozygous dominant for the trait of height ( ) with a plant that is homozygous dominant for that trait ( )
2. Cross a plant that is homozygous dominant for the trait of height ( ), with a plant that is heterozygous for that trait ( ).
3. Cross a plant that is heterozygous for the trait of height ( ), with a plant that is heterozygous for that trait ( ).
Testcross. *A testcross is the procedure in which an individual of unknown genotype is crossed with a homozygous recessive individual. *A testcross can determine the genotype of any individual whose phenotype is dominant. *For example, in rabbits, both of the genotypes BB and Bb result in a black coat. A black rabbit is found in your garden and you want to find out whether it is homozygous dominant (BB), or heterozygous (Bb). *You simply breed the mystery rabbit with a homozygous recessive rabbit.
Determine the genotype of an unknown black rabbit by performing a test cross.
Incomplete dominance. *Incomplete dominance occurs when two or more alleles influence the phenotype. *This result in a blending of traits in the offspring’s phenotype. *In Japanese four o’clock plants, both the allele for red flowers (R) and the allele for white flowers (R’) influence the phenotype, but neither allele is dominant. *RR results in red flowers. *R’ R’ results in white flowers. *RR’ results in pink flowers which is a blending of the red and white traits.
CROSS A RED 4 O’CLOCK PLANT WITH A WHITE 4 O’CLOCK PLANT. LIST THE OFFSPRING
Dihybrid Crosses. *A dihybrid cross involves two pairs of contrasting traits, instead of one trait as in a monohybrid cross. *Predicting the results of a dihybrid cross is more complicated than predicting the results of a monohybrid cross because there are more possible combinations of alleles to work out. *A dihybrid cross such as TtRr X TtRr will yeild 16 offspring. *A heterozygous X heterozygous dihybrid cross always results in a 9:3:3:1 phynotypic ratio in the offspring.