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 Number of limbs (an accident causes loss of leg), plastic surgery to change original nose shape, etc.

Dogs Tell your CAT how to differentiate between a characteristic and a trait Number of limbs (an accident causes loss of leg), plastic surgery to change original nose shape, etc.

How do we inherit traits from our parents? Remember Meiosis? Chromosomes carry the genes (alleles). Chromosomes duplicate and homologous pairs line up and make tetrads. One may carry a dominant allele and the other may carry a recessive allele. In meiosis I, the homologous pairs (tetrads) separate. In Meiosis II, the sister chromatids (duplicates) separate. Giving each of the 4 gametes an allele. Whichever gamete fertilizes is the allele baby will receive.

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 – share Cats 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? b B Bb No, the results are the same.

Monohybrid Cross Monohybrid = a cross between two organisms that predicts the inheritance pattern/probability of only one characteristic at a time

Practice Together BB Bb BB Bb 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? Brown Fur = B BB and Bb B b BB Bb B B BB Bb

Check for understanding 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? Two-eyed 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 b b B Bb Bb Bb Bb 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 B Bb Bb Bb Bb

Heterozygous x heterozygous 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! B b 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 b b b b B Bb Bb B b Bb Bb bb Bb Bb bb 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 b 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