Genetics 7 th Grade Mrs. Boguslaw
Introduction and Background Genetics = the study of heredity During meiosis, gametes receive ½ of their parent s chromosomes During sexual reproduction, two gametes (male and female) come together to form a new organism This organism has one set of DNA from mom and one set of DNA from dad 23 chromosomes from mom + 23 chromosomes from dad = full set of 46 chromosomes (23 pairs!) Allele = different forms of a gene
Gregor Mendel The Father of Genetics Austrian monk that studied and experimented with pea plants Looked at characteristics like flower color, plant height, pea color, and pea shape Developed important laws of genetics Parent x Parent à F1 Generation à F2 Generation
Three generations of data 1. P Generation (Parents) - Cross two purebred plants 2. F1 Generation (Offspring generation #1) - All plants looked the same - Same traits expressed in all of the F1 generation 3. F2 Generation (Offspring generation #2) - The missing trait reappeared
What did this look like? Example: Pea Plant Height P Generation Pure tall x Pure short F1 Generation All look the same; 100% tall F2 Generation Short trait comes back! 75% tall, 25% short
Alleles Different versions of the same gene Located on homologous chromosomes Can be dominant or recessive Dominant: ALWAYS expressed Recessive: can be masked by a dominant allele
Allele Combinations Homozygous Dominant Heterozygous Same alleles = same case letters Dominant is capitalized TT, PP, GG, DD, etc. Different alleles = different case letters One capital, one lowercase Tt, Pp, Gg, Dd, etc. Homozygous Recessive Same alleles = same case letters Recessive is lower case tt, pp, gg, dd, etc.
Genotype vs Phenotype Genotype the GENES the organism has the specific letters (alleles) that an organism has Note the ratio! Phenotype the PHYSICAL APPEARANCE of an organism the expression of the genes Note the ratio!
Punnett Squares Diagrams to predict the outcomes of genetic crosses Can be done for one trait (monohybrid) or two traits (dihybrid) Typical setup: female parent alleles on left side, male parent alleles on top
How to draw a Punnett square Monohybrid (one trait) : 4 squares Dihybrid (two traits) : 16 squares http://www.youtube.com/watch?v=prkhkjfumms
Incomplete Dominance Both alleles are expressed, to some degree If heterozygous, BOTH traits are expressed and blended together
Incomplete Dominance in People Hair Type Curly, Wavy, Straight Voice pitch in men high, medium, low Some genetic diseases have some degree of incomplete dominance Tay-Sachs (autosomal recessive): abnormal enzyme activity in heterozygous carriers
Codominance Neither allele is recessive both are expressed fully Human example: blood type AB Common other examples:
Sex-linked
Lets recall how we determine gender How do we determine the sex of an individual? à through sex chromosomes Sex Chromosomes: X and Y Female: XX Male: XY Who is responsible for gender determination in the child? The father because he gives an X or Y to the gametes. The mother only gives an X to the gamete Which sex chromosome is bigger? à the X chromosome (y is much smaller)
What are sex-linked genes? à genes found on a sex chromosome X-linked genes are genes found on the X chromosome, symbolized by X r, X R, Y. Y-linked genes are found on the Y chromosome, symbolized by X, Y R, Y r Thomas Morgan experimented with the eye color of fruit flies (Drosophilia) to determine X- linkage
X-linked Genes X-linked recessive traits will show up more often in males because they have only 1 X chromosome Let s do a Punnett square for a female carrier of an x-linked gene and a recessive male What will the genotypes be? X R X r and X R Y
Here s the results For girls: 0% have the trait For boys 50% have it. Probability is higher for boys because whatever X they get determines the trait, for girls they have to get 2 recessive X s. X R X r X R X R X R X R X r Y 0 X R Y 0 X r Y 0
Multiple Alleles More than two possibilities for alleles (more than just the dominant/recessive patterns we ve seen so far!) Example: A, B, O blood types Side note: blood types also show codominance (AB)!
Polygenic Traits Two or more genes interact to form a trait Can be influenced by environment (i.e. Nutrition) Examples include eye color, height, skin color, etc.
Pedigrees
What is a pedigree chart? Pedigree charts show a record of the family of an individual They can be used to study the transmission of a hereditary condition They are particularly useful when there are large families and a good family record over several generations. 2007 Paul Billiet ODWS
Studying human genetics Pedigree charts offer an ethical way of studying human genetics A genetic counselor will still use pedigree charts to help determine the distribution of a disease in an affected family 2007 Paul Billiet ODWS
Symbols used in pedigree charts Normal male Affected male Normal female Affected female Marriage A marriage with five children, two daughters and three sons. The middle son is affected by the condition. Eldest child Youngest child 2007 Paul Billiet ODWS
Organizing the pedigree chart A pedigree chart of a family showing 20 individuals 2007 Paul Billiet ODWS
Organizing the pedigree chart Generations are identified by Roman numerals I II III IV 2007 Paul Billiet ODWS
Organizing the pedigree chart Individuals in each generation are identified by Arabic numerals numbered from the left Therefore the affected individuals are II3, IV2 and IV3 I II III IV 2007 Paul Billiet ODWS
The inheritance pattern of a trait can often be determined by looking at a pedigree
Karyotypes A karyotype is a picture of all the chromosomes in a cell Karyotypes can be used to detect genetic problems we can see if there is an extra chromosome, a missing chromosome, or a damaged or incomplete chromosome X Y
Normal Human Karyotype Gender?
Normal Human Karyotype Gender?
Can you spot any potential abnormalities?
Cri-du-chat Syndrome