Genetics and Heredity 9 Biology

Terminology Chromosome Allele Homozygous Heterozygous Dominant Recessive Monohybrid Dihybrid Incomplete dominance Gene Gamete XX, XY Phenotype Genotype Karyotype Mutation Autosomes

Chromosomes Contain genetic information Made of DNA Humans have 23 pair Undergo mitosis Somatic cells, autosomes Undergo meiosis Gametes, sex cells, sperm and egg

Genes Encode for traits of an organism Alternate form of a gene is an allele You have 2 alleles for each trait One from your mother One from your father

Alleles Alleles can be dominant or recessive Your genotype contains all the alleles that you carry Your phenotype is the visible evidence of those alleles

Example of Allele Dominance Eye color: Brown eyes are dominant over lighter colored eyes In order for brown eyed parents to have a blue eyed child, they both must carry the recessive allele for blue eyes

Dominant vs. Recessive Dominant alleles are expressed in the organism’s phenotype (visible) Recessive alleles are only expressed if the organism has 2 copies of the recessive allele

Incomplete Dominance Incomplete dominance results in an intermediate phenotype Ex: Red flowers and white flowers When red and white flowers are crossed, their offspring are pink

Mendelian Genetics and Monohybrid Crosses Gregor Mendel Monk who experimented with crosses of pea plants Considered the “Father of Genetics” Worked out genetic probabilities using Punnett squares Worked with monohybrid and dihybrid crosses

Homozygous and Heterozygous Both alleles are the same Ex: AA or aa AA indicates the dominant allele is expressed in phenotype aa indicates the recessive allele will be expressed in phenotype Heterozygous: Both of the alleles are different Ex: Aa Aa indicates the dominant allele will be expressed in phenotype, but the individual is a carrier of the recessive allele

Monohybrid Crosses Using Punnett Squares Crosses that involve one trait Assign letters indicating dominant and recessive alleles Ex: B (dominant purple) b (recessive white) Female genotype Bb Male genotype Bb

Monohybrid Crosses Using Punnett Squares, cont. Results: Offspring genotypes: BB, Bb, Bb, bb Offspring phenotypes: 3 purple petals 1 white petals Homozygous: 2 alleles that are the same (BB or bb) Heterozygous: 2 alleles that are not the same ( Bb)

Monohybrid Practice:

Ratios and Percentages Interpretation of genetic crosses often require either a ratio or a percentage. Genotypic ratio: 1:2:1 (1 BB, 2Bb, 1 bb) Phenotypic ratio: 3:1 (3 purple, 1 white) Genotypic percentage: 25% BB, 50% Bb, 25% bb Phenotypic percentage: 75% purple, 25% white

Dihybrid Crosses Dihybrid crosses use 2 traits They also use Punnett squares, but have 16 squares instead of 4 Example: Seed color/ seed texture Parent 1 RrYy Parent 2 RrYy Phenotype for both: yellow, smooth

Setting Up Dihybrid Crosses 1. Identify genotype of the parents. RrYy and RrYy Write: 1 2 3 4 & 1 2 3 4 R r Y y & R r Y y Use this series of pairings to label your dihybrid Punnett square: 1-3, 1-4, 2-3, 2-4 1-3= RY 1-3= RY 1-4= Ry 1-4= Ry 2-3= rY 2-3= rY 2-4= ry 2-4= ry

Interpreting Dihybrid Crosses Genotypes: RRYY ( 1 ) RRYy ( 2 ) RrYY ( 2 ) RrYy ( 4 ) Rryy ( 3 ) rrYY ( 3 ) rryy ( 1 )

Interpreting Dihybrid Crosses, cont. Phenotypes: Yellow, round = 9 Green, round = 3 Yellow, wrinkled = 3 Green, wrinkled = 1 Phenotypic ratio: 9:3:3:1 Classic ratio for the dihybrid cross of 2 heterozygotes

Practice!

XX and XY Females are genetically XX Female egg carries only X chromosomes Males are genetically XY Male sperm carries both X and Y chromosomes Male sperm determines the sex of the baby The sex chromosomes are the last pair of chromosomes in a karyotype. The X chromosome is quite large, and the Y chromosome is quite small

X and Y Chromosomes

Karyotypes Karyotypes are photographs of chromosomes Chromosomes are matched by hand or computer, in pairs depending on their size, and centromere placement Karyotypes can tell us: If the number of chromosomes is normal If the fetus will have a genetic defect of chromosome number Karyotypes cannot tell us: the degree of limitations the fetus may have

Sex-Linked Disorders Carried only on X chromosome Because of incomplete masking of the X chromosome by the Y, males are more frequently affected. Examples of Sex-Linked Disorders: Color-blindness Hemophilia Duchenne's Muscular Dystrophy

Co-Dominance Co-dominance occurs when neither allele is dominant Both alleles are expressed Example: Blood Typing There are 4 blood types: A, AB, B, O The alleles are: IA, IB, i Combinations are: IAIA, IAi, IAIB, IBIB, IBi, ii Blood types are either Rh+ or Rh– Rhesus factor: a protein that is either present, or not present

https://www.youtube.com/watch?v=xfZhb6lmxjk

Practice Blood Type Crosses