Mendel’s Laws, Monohybrid and dihybrid crosses Mrs. Stewart Honors Biology
Bell work Describe the relationship between genotype and phenotype
Standards • CLE 3210.4.1 Investigate how genetic information is encoded in nucleic acids. • CLE 3210.4.3 Predict the outcome of monohybrid and dihybrid crosses.
objectives • Analyze the law of segregation • Create a punnett square using the genotypes of parents • Predict the outcome and probability of monohybrid crosses
Decide with your partner
Review: What is Heredity? • Why do children look like their parents? • Why do brothers and sisters resemble each other? • We inherit traits from our parents • Heredity = the passing of genetic traits from parents to offspring
Traits and genes • Genes carry the instructions that define our traits • Genes = segments of the DNA sequence that code for a particular trait • Traits = genetically determined variations of characteristics (qualities) • Example: natural hair color, eye color, skin tone, etc. • The environment we live in can also help define our traits • Example: a person’s genes may code for a certain hair color, but exposure to dyes, chemicals, sunlight, etc can change that color • Characteristic = can be altered by the environment
Dogs • Tell your CAT how to differentiate between a characteristic and a trait
How do we inherit traits from our parents? • Remember Meiosis?
How do we inherit traits from our parents? • Human body cells (somatic cells) have 2 complete sets of 23 chromosomes • 2 x 23 = 46 chromosomes • One set of 23 comes from sperm (Dad) • One set of 23 comes from egg (Mom) • Each parent contributes one complete set to the child, giving the child a “mix” of genes
Fertilization • Fertilization – one sperm fuses with an egg to form a zygote • The zygote now has 2 sets of 23 chromosomes (46 total) • This cell will begin dividing and will ultimately become a child. Zygote
cats • Tell your DOG how we inherit “traits” from our parents
Parents contribute one of each chromosome pair to the child • On a karyotype, there are 2 chromosomes at each site. • These represents the 2 chromosomes received from the parents. One from mom, one from dad. • During meiosis, these will separate into different gametes (sex cells).
Mendel proposed two laws • These laws explain how the homologous chromosome pairs for each parent will separate into the gametes during meiosis. • Law of segregation • Law of Independent Assortment
Law of segregation • Homologous chromosomes separate during the formation of gametes
Dogs • Explain the law of segregation to your CAT
Law of Independent Assortment • Allele pairs separate independently during gamete formation -which means that the transmission of traits to offspring are independent to one another.
Cats • Explain the law of independent assortment to your DOG
Siblings • Since parents contribute chromosomes randomly, every child inherits a “unique” combination of traits. • Some may resemble mom; some may resemble dad; others will be completely unique • They may be resemble each other or be totally different.
Think – pair – shareCats and Dogs • How can we predict the inheritance of traits? Punnett Squares
Punnett Squares? • Punnett Squares use genotypes to predict inheritance • Punnett Squares show the law of segregation in action • What do the letters on the outside of the punnett square represent? Mom Dad • What do each of the squares inside the punnett square represent?
Does it matter which side you put the parent’s genotype on? No, the results are the same.
Monohybrid Cross • Monohybrid = a cross between two organisms that predicts the inheritance pattern/probability of only onecharacteristic at a time
Practice Together Brown Fur = B • fur color B = brown fur b = white fur • Which trait is dominant? • Cross a homozygous dominant with a heterozygous dominant • What are the genotypes of the parents? • What percentage of the offspring will have white fur? BB and Bb 0 B b BB Bb B B BB Bb
Check for understanding Two-eyed • A one-eyed purple people eater is crossed with a two-eyed purple people eater. All of their offspring have two eyes. Which trait is dominant? • Use the letter E or e to represent the alleles (variations) for this gene. What is the genotype of the offspring if you cross a purebred one-eyed purple people eater with a homozygous two-eyed purple people eater? • What generation are the offspring of this cross part of? • If you crossed the offspring with each other, how many of the resulting offspring would have two eyes? Ee F 1 3 out of 4 or 75 %
Ratios • Ratio– how much of one thing there is in comparison to another • Genotypic ratio = ratio of possible genotypes in offspring of a cross • Phenotypic ratio = ration of possible phenotypes in offspring of a cross
Known Shortcuts • Homozygous dominant crossed with a homozygous recessive • 100% of offspring will be heterozygous • 4:0 ratio of heterozygous to other genotypes • 100% of offspring will show dominant trait • 4:0 ratio of dominant to recessive • Example: Cross a BB x bb B = black fur; b = white fur b b Bb B B Bb Bb Bb
Heterozygous x heterozygous B b • Complete this cross - Bb x Bb • Genotypic ratio: • BB : Bb: bb = 1:2:1 Always! • Phenotypic ratio: • Dominant trait shown: Recessive Trait shown = 3:1 Always! BB Bb B b Bb bb
Test Cross • Guinea Pig Fur Color B = Black fur b = White fur • What are the possible genotypes for a Guinea Pig with black fur? • How can we determine which genotype is correct? • Test Cross = cross an individual that expresses the dominant trait with a homozygous recessive individual to determine the genotype
Test Cross • If a black guinea pig is crossed with a homozgous recessive white guinea pig and even one of the offspring is white, what is the only possible genotype for the black guinea pig? b b bb B B Bb Bb B b Bb Bb bb Bb Bb bb
Three types of dominant relationships for alleles • Complete dominance • Incomplete dominance • Codominance
Complete Dominance • Standard dominant vs recessive relationship
Incomplete dominance • One is not completely dominant over the other • Instead, heterozygotes will have a blending of the dominant and recessive traits
Codominance • Both alleles for a gene are expressed in the heterozygotes – simultaneously • Neither allele is dominant or recessive, nor do they blend together
Dihybrid Crosses • A cross in which TWO characteristics are tracked
Use f.o.i.l. method to determine gametes
F – pair the first two alleles of each characteristic • O – pair the outer two alleles • I – pair the inner two alleles • L – Pair the last two alleles for each characteristic • F – BL O – Bl I – bL L – bl • You do the next one on your worksheet
Heterozygous x heterozygous • What are the gamete combinations for each guinea pig? • Offspring Shortcut: 9:3:3:1 phenotypic ratio Always! • 9 – Two dominant traits • 3 – One dominant one recessive trait • 3 – The other dominant and recessive combo • 1 – both recessive traits
