Patterns of Inheritance Mendelian Inheritance

Key concepts Mendel’s research showed that traits are inherited as discrete units. Mendel’s data revealed patterns of inheritance The same gene can have many versions Genes influence the development of traits The inheritance of traits follow the rules of probability A monohybrid cross involves one trait A dihybrid cross involves two traits.

Case study: Sudden Death on the Court http://www.clevelandleader.com/node/6446 Michael Phelps' Read Sudden death on the court of Flo Hyman Handout to be made from the ninth Edition Share your understanding of genetic disorder with your partner

What is the Physical Basis of Inheritance? Genes are sequences of Nucleotides at specific locations on chromosomes. A chromosome consists of a single double helix of DNA made up of nucleotides. Genes the units of inheritance are parts of chromosomes. A gene’s physical location on a chromosome is called locus. The chromosomes of diploid organisms occur in pairs called homologues.

Alleles Both members of a pair of homologues carry the same genes, located at the same loci. However, the nucleotide sequences of a given gene may differ in different members of a species, or even on the two homologues of a single individual. These different versions of a gene at a given locus are called alleles. The relationships among genes, alleles, and chromosomes can be seen in the next slide.

Both chromosomes carry the same allele of the gene at this locus, and so the organism is homozygous at this locus Each chromosomes carries a different allele of this gene, so the organism is heterozygous at this locus

Mutations are the source of Alleles Alleles arise as mutations that slightly change the nucleotide sequences. If a mutations occurs in the cells that become sperm or eggs, it can be passed on from parent to offspring. Most of the alleles in an organism’s DNA first appeared as mutations in the reproductive cells of the organism’s ancestors, perhaps millions of years ago, and have been inherited generations after generations.

Homozygous and Heterozygous A diploid organism has pairs of homologous chromosomes, and both members of a pair contain the same gene loci and so the organism has two copies of each gene. If both homologues have the same allele at a given gene locus, the organism is said to be homozygous at that locus If two homologous chromosomes have different alleles at a locus, the organism is heterozygous at that locus. The organism that are heterozygous at a specific locus are often called hybrids

Homozygous and Heterozygous

The Secrets of Mendel’s Success The three key steps to Mendel’s Success Choosing the right organism with which to work Designing and performing the experiment correctly Analyzing the data correctly

Parts of edible pea flower Useful link to know about Mendel’s Experiment

Edible Pea flower Mendel chose the edible pea as the subject for his experiments. In pea flowers, the petals enclose all of the reproductive structures, preventing another flower’s pollen from entering. Therefore the egg cells in a pea flower must be fertilized by sperm from the pollen of the same flower and this process is called self-fertilization. Mendel also opened the pea flower and removed its stamens, preventing self fertilization. Then he dusted the sticky tip of the carpel with pollen from the flower of another plant and this is cross fertilization

Traits of Pea Plants studied by Mendel Mendel chose to study individual characteristics (called traits) that had different forms such as white versus purple flowers. He started out to study one trait at a time. He followed the inheritance of these traits for several generations, counting the number of offspring with each type of trait. By analyzing these numbers, the basic patterns of inheritance became clear

The genetic constitution of an organism Examples: TT, Tt, tt. 4.3.1 Define genotype, phenotype, dominant allele, recessive allele, recessive allele, codominant alleles, locus, homozygous, heterozygous, carrier, and test cross Genotype: Genotype: The symbolic representation of a pair of alleles possessed by an organism, typically represented by two letters. The genetic constitution of an organism Examples: TT, Tt, tt. Phenotype Phenotype: The characteristics or trait of an organism- the way in which the genotype is expressed (appearance of an organism) Examples: five fingers on each hand, color blindness.

Definitions Dominant allele An allele that has the same effect on the phenotype whether it is paired with the same allele or a different one. Dominant alleles are always expressed in the phenotype. An allele that has the same effect on the phenotype in a heterozygous individual (where it is combined with a recessive allele) as in a homozygous individual (where there are two copies of the dominant allele) Recessive allele: an allele that has an effect on the phenotype in homozygous individuals (where there are two copies of the recessive allele). In heterozygous individuals the recessive allele is hidden by the dominant allele.

- Homozygous: having two identical alleles of a gene. Examples: TT is a genotype which is homozygous dominant whereas tt is the genotype of someone who is homozygous recessive for that trait. Heterozygous: Having two different alleles of a gene. Example: Tt is a heterozygous genotype. Test cross: testing a suspected heterozygote by crossing it with a known homozygous recessive.

Definitions Codominant alleles: pairs of alleles that both affect the phenotype when present in a heterozygote. Locus: the particular position on homologous chromosome of a gene. Each gene is found at a specific place on a specific pair of chromosome. So alleles of the same gene occupy the same locus.

Monohybrid Cross Animation http://www.siskiyous.edu/class/bio1/genetics/monohybrid_v2.html

MonoHybrid Crosses: a cross involving one trait Experiment: Crossing a homozygous purple flowered plant with a homozygous white flowered plant Appearance = phenotype Genetic make-up = genotype F1 = a hybrid formed by crossing differing pure-breeding parents.   Homozygous = both alleles the same. Hetrozygous = both alleles the different. F2 = the offspring produced by inbreeding the F1.

4.3.2 Determine the genotypes and phenotypes of the offspring of a monohybrid cross using a Punnet grid.

Monohybrid Cross We can study one of the characteristics Gregor Mendel used in his experiments. He studied the size of pea plants and found that ‘tall’ is dominant over ‘short’. If we start the experiment with 2 pure breeding (homozygous) plants of contrasting traits (tall and short), we will obtain an F1 (First Filial generation) which has the dominant phenotype (tall) but is heterozygous. When self-fertilizing the F1, we will obtain an F2 (Second filial generation) which will appear ¾ dominant (tall) and ¼ recessive (short) Start by writing the phenotypes for the relevant gene and what the corresponding genotypes are as in the following table. Possible phenotypes Corresponding genotypes Tall TT or Tt Short Tt Then write out the cross. Make sure to include both genotypes and phenptypes of the parents (P), as well as the genotype and phenptype of the offsproing (F1).

Monohybrid Cross http://www.jdenuno.com/PDFfiles/Monohybrid.pdf You can go to this site and have practice for Monohybrid cross using Punnet grid.

Drag and drop Genetics Monohybrid http://www.zerobio.com/drag_gr11/mono.htm Practice Monohybrid cross by using this link

Incomplete Dominance

Incomplete Dominance When the heterozygous phenotypes is intermediate between the two homozygous phenotypes, the pattern of inheritance is called incomplete dominance. In humans, hair texture is influenced by a gene with two incompletely dominant alleles which we call C1 and C2. A person with two copies of the Copies of the C1 allele has curly hair; two copies of C2 allele produce straight hair. Heterozygotes, with C1C2 genotype have wavy hair. Work out the cross between two parents having wavy hair and show parental, F1, and F2 offspring.

Laws of Mendel Law of Segregation The pairs of alleles on homologous chromosomes separate, or segregate, from each other during meiosis. As a result, each gamete receives only one allele of each pair Mendel's law of independent assortment, states that allele pairs separate independently during the formation of gametes. This means that traits are transmitted to offspring independently of one another.

http://www.siskiyous.edu/class/bio1/genetics/dihybrid_v2.html Animation on Dihybrid cross

Dihybrid cross genetic diagram F1 Phenotypes RrYy (Selfed) Genotypes Round Yellow Proportions 100% meiosis Gametes Y y R RY Ry r rY ry

Dihybrid cross genetic diagram Gametes Y y R RY Ry r rY ry fertilisation F2 Genotypes RRYY RRYy RrYY RrYy RRyy Rryy rrYY rrYy rryy

Dihybrid cross proportions Phenotypes Proportions Round Yellow 9/16 or 56.25% Round Green Wrinkled Yellow Wrinkled Green

Dihybrid cross proportions Phenotypes Proportions Round Yellow 9/16 or 56.25% Round Green 3/16 or 18.75% Wrinkled Yellow Wrinkled Green 1/16 or 6.25%

Dihybrid test cross In monohybrid crosses, to know if a dominant trait is homozygous (RR) or heterozygous (Rr) it is necessary to carry out a test cross This is done with a homozygous recessive (rr) individual The same is true for a dihybrid cross where the test cross is made with an individual which is homozygous recessive for both characters (rryy)

Dihybrid test cross Phenotypes Round Yellow X Wrinkled Green Genotypes RrYy rryy Gametes RY, Ry, rY, ry ry Genotypes RY Ry rY ry RrYy Rryy rrYy rryy Phenotypes Round Yellow Round Green Wrinkled Yellow Wrinkled Green Proportions 1/4 or 25%

Dihybrid Cross Problem In summer squash, white fruit color (W) is dominant over yellow fruit color (w) and disk-shaped fruit (D) is dominant over sphere-shaped fruit (d)..  If a squash plant true-breeding for white, disk-shaped fruit  is crossed with a plant true-breeding for yellow, sphere-shaped fruit,  what will the phenotypic and genotypic ratios be for: a. the F1 generation?     b. the F2 generation? http://www.personal.psu.edu/hw7/Biology110/dihybrid.htm You can check your answer

Useful links and reference http://www.siskiyous.edu/class/bio1/genetics/dihybrid_v2.html Animation on Dihybrid cross http://www.zerobio.com/drag_gr11/mono.htm Drag and drop Genetics Monohybrid http://www.dnalc.org/resources/animations/ Animations on Genetics