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Monohybrid Crosses Objective

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1 Monohybrid Crosses Objective
Use the punnett square method to determine the possible genotypes of different monohybrid crosses Determine the phenotypic ratio of a monohybred cross. Explain how the process of meiosis and fertilization account for the transmission of inherited characteristics.

2 Inheritance - Mendel The transfer of characteristics (traits) from one generation to another Your inherited characteristics (traits) are determined by the genes located on your chromosomes

3 # of students with Dominant # of students with Recessive Ratio of
Dominant Trait Description # of students with Dominant # of students with Recessive Ratio of Dominant to Recessive Percentage with Recessive traits Class Eye colour Hair colour Hairline Freckles Earlobe Fingers Ear rim Thumb joint Folded hands Tongue rolling Chin dimple

4 Gene a short segment of a chromosome coding for one trait

5 Homologous chromosomes:
chromosomes are found in pairs in humans, there are 46 chromosomes, made up of 23 pairs of homologous chromosomes a pair of chromosomes consists of two homologous chromosomes which look alike and carry genes for the same traits you receive one homologous chromosome of the pair from each parent, therefore you receive 2 genes for each trait

6 Phenotype This term is used to describe the physical or visible appearance of an individual as determined by the gene combination you inherit from your parents. Example: in the case of eye colour, having brown eyes is the phenotype and is dominant

7 Genotype the gene combination that produces the trait
these genes may be the same or different genotypes are represented by upper and/or lower case letters e.g. Bb there are three possibilities for genotypes for a single trait. e.g. BB or Bb or bb

8 Allele alternate forms of a gene
same location on a each chromosome of pair affects the same trait but differently alleles are different forms of a gene that carry different instructions eg. Brown eyes or blue eyes


10 Dominant Trait/Allele
Characteristic that is always expressed its gene is present only one dominant allele needs to be present for the dominant trait to be expressed an upper case letter designates a dominant allele eg. Tall is dominant. It is represented with a “T”

11 Recessive Trait/Allele
Characteristic that is only expressed when two genes for that trait are present two recessive alleles need to be present for a recessive trait to be expressed if a dominant allele is present it “masks” the recessive allele eg: Short is recessive. It is represented with a “t”

12 Homozygous Genotype “homo” means the “same”
the condition where both alleles for a trait are the same there are two conditions for homozygous genotype: homozygous recessive genotype “aa” homozygous dominant genotype “AA”

13 Heterozygous Genotype
“hetero” means “different” the condition where both alleles for a trait are different the genetic information inherited for a trait from both parents is different example: heterozygous genotype “Aa”

14 Summary (example eye colour)
Genotype Condition Phenotype BB Homozygous dominant Brown eyes Bb Heterozygous bb Homozygous recessive Blue eyes

15 Pure breeding plants always produce identical offspring.
Mendel’s Experiment Crossed two purebred parents (One tall -T and one short -t) Pure breeding plants always produce identical offspring. All offspring were tall!! Mendel concluded that some traits were ___________ and some traits were ___________.

16 Principal of dominance
When individuals with different traits are crossed, the offspring (F1 generation) will express only the dominant trait. Parents: Tall x Short F1 (Offspring): Tall Tall Tall Tall What must the genotype of the offspring be if the parents were pure breeding?

17 Hybrids have contrasting traits.
What happened next?? Mendel crossed the F1 generation. He crosses two hybrid pea plants. Hybrids have contrasting traits. F1: Tall plants x Tall plants F2 (Offspring of F1): Tall Tall Tall Short F2 generation = 3:1 ratio What must the genotype of these offspring be if the parents were hybrids (Tt and Tt)?

18 Law of Segregation Each F1 parent starts with two hereditary factors (alleles); one is dominant and one is recessive Each parent contributes only one factor (allele) Each offspring inherits one factor (allele) from each parent If the dominant factor (allele) is present it will be expressed. If the recessive factor (allele) is present it will only be expressed if only recessive factors are present.

19 Monohybrid Cross Mendel crossed two purebred plants TT – tall plant
Tt – short plant Monohybrid cross – only one trait is being tested

20 Punnett Squares Punnett Square
Is a grid system resembling a checkerboard, used in computing possible results of various genetic combinations Simply stated, it is a way of representing the possible combinations of genes when an egg and sperm unite in fertilization

21 Predicting genotype and phenotype possibilities using Punnett Squares
 Characteristic hair texture – Dominant allele (gene) is curly hair - C. Recessive allele (gene) is straight hair - c Mother Phenotype – straight hair Genotype- homozygous recessive - cc Father Phenotype – curly hair Genotype – homozygous dominant – CC

22 Punnett Square Cc c C Egg Sperm
All of their children will have curly hair. They all have a different genotype from their parents.

23 Fig. 18.5 Fig. 18.5

24 Fig. 18.6 Fig. 18.6

25 Fig Fig

26 Fig Fig

27 Your Turn Bikini Bottom Genetics – Monohybrid Crosses
HOMEWORK: Find out what the following words mean… Hybrid Pure-breeding/Purebred Why was it important that Mendel used purebred plants in his experiment? How does meiosis ensure that you inherit your mothers and fathers characteristics?

28 Codominance, Incomplete dominance and Dihybrid Crosses
Objectives: Explain what co dominance and incomplete dominance is. Give an example of each. Use a Punnett square to solve basic incomplete and co dominance crosses Use a Punnett square to solve basic dihybrid crosses.

29 Terms Incomplete Dominance: When two alleles are equally dominant, they interact to produce a new phenotype. Codominance: When both alleles are dominant and are expressed at the same time. Pg 145 # 1-4

30 Codominance When a red bull is crossed with a white
cow, the offspring will be roan.

31 Incomplete Dominance When a red flower is crossed with a red flower,
the resulting offspring will be pink.

32 Dihybrid Crosses A type of cross that involves two genes, each consisting of non identical alleles.

33 What happened when Mendel crossed two pea plants that different in TWO traits?
In his second experiment Mendel crossed a pea plant with round/yellow (RRYY) seeds with a pea plant with wrinkled/greed seed (rryy). RY ry RrYy RY ry Genotype: RrYy F1 - Generation Ratio 100% Phenotype: Round/Yellow Ratio 100% What genotypes resulted? What phenotypes resulted?

34 Crossing F1(RrYy) x F1(RrYy)
Genotype: RRYY RRYy Rryy RrYY RrYy rrYY rrYy rryy Ratio: Phenotype: Round/Yellow Round/Green Wrinkled/Yellow Wrinkled/Green Ratio: In the resulting F2 – Generation, the phenotypic ratio will always be 9:3:3:1 In other words, the likelihood of obtaining each of the above phenotypes from crossing two hybrid pea plants is 9:3:3:1. What genotypes resulted? What phenotypes resulted?

35 Law of Independent Assortment
The inheritance of alleles for one trait ________ (does/does not) affect the inheritance of alleles for another trait.

36 Plenary Bikini Bottom Questions

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