Introduction to genetics

Mendels Laws, Monohybrid and dihybrid crossesMrs. StewartHonors BiologyBell workDescribe the relationship between genotype and phenotypeStandardsCLE 3210.4.1Investigate how genetic information is encoded in nucleic acids.

CLE 3210.4.3 Predict the outcome of monohybrid and dihybrid crosses.

objectivesAnalyze the law of segregationCreate a punnett square using the genotypes of parentsPredict 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

6Traits and genesGenes carry the instructions that define our traitsGenes = segments of the DNA sequence that code for a particular traitTraits = 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 traitsExample: a persons genes may code for a certain hair color, but exposure to dyes, chemicals, sunlight, etc can change that colorCharacteristic = can be altered by the environmentNumber of limbs (an accident causes loss of leg), plastic surgery to change original nose shape, etc. 7DogsTell 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. 8How 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. 9How do we inherit traits from our parents?

Human body cells (somatic cells) have 2 complete sets of 23 chromosomes 2 x 23 = 46 chromosomesOne 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 genesFertilizationFertilization one sperm fuses with an egg to form a zygoteThe zygote now has 2 sets of 23 chromosomes (46 total)This cell will begin dividing and will ultimately become a child.

ZygotecatsTell your DOG how we inherit traits from our parents

12Parents contribute one of each chromosome pair to the childOn 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 lawsThese laws explain how the homologous chromosome pairs for each parent will separate into the gametes during meiosis.

Law of segregation

Law of Independent AssortmentLaw of segregationHomologous chromosomes separate during the formation of gametes

DogsExplain the law of segregation to your CAT

Law of Independent AssortmentAllele pairs separate independently during gamete formation -which means that the transmission of traits to offspring are independent to one another.

CatsExplain the law of independent assortment to your DOG

SiblingsSince parents contribute chromosomes randomly, every child inherits a unique combination of traits.Some may resemble mom; some may resemble dad; others will be completely uniqueThey may be resemble each other or be totally different.

Think pair shareCats and DogsHow can we predict the inheritance of traits?

Punnett SquaresPunnett Squares?Punnett Squares use genotypes to predict inheritancePunnett Squares show the law of segregation in action

MomDadWhat do the letters on the outside of the punnett square represent?

What do each of the squares inside the punnett square represent?

Does it matter which side you put the parents genotype on?

No, the results are the same.Monohybrid CrossMonohybrid = a cross between two organisms that predicts the inheritance pattern/probability of only one characteristic at a timePractice Togetherfur color B = brown fur b = white furWhich trait is dominant?Cross a homozygous dominant with a heterozygous dominantWhat are the genotypes of the parents?What percentage of the offspring will have white fur?

Brown Fur = B

BB and BbBbBBBBBBBbBb0Check for understandingA 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-eyedEeF 13 out of 4 or 75 %

RatiosRatio 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 ShortcutsHomozygous dominant crossed with a homozygous recessive100% of offspring will be heterozygous4:0 ratio of heterozygous to other genotypes100% of offspring will show dominant trait4:0 ratio of dominant to recessiveExample: Cross a BB x bb B = black fur; b = white fur

BbBbBbBb bbB

BHeterozygous x heterozygousComplete 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 BbBbBbBBbbTest CrossGuinea Pig Fur ColorB = 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 genotypeTest CrossIf 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?

BbBbBbBb bbB

BBbBbbbbbbbB

bThree types of dominant relationships for allelesComplete dominance

Incomplete dominance

CodominanceComplete DominanceStandard dominant vs recessive relationship

Incomplete dominanceOne is not completely dominant over the otherInstead, heterozygotes will have a blending of the dominant and recessive traits

CodominanceBoth alleles for a gene are expressed in the heterozygotes simultaneouslyNeither allele is dominant or recessive, nor do they blend together

Dihybrid CrossesA cross in which TWO characteristics are tracked

Use f.o.i.l. method to determine gametes

F pair the first two alleles of each characteristicO pair the outer two allelesI pair the inner two allelesL Pair the last two alleles for each characteristic

F BLO Bl I bLL bl

You do the next one on your worksheet

Heterozygous x heterozygousWhat are the gamete combinations for each guinea pig?Offspring Shortcut:9:3:3:1 phenotypic ratio Always!

9 Two dominant traits3 One dominant one recessive trait3 The other dominant and recessive combo1 both recessive traits