monohybrid crosses
DESCRIPTION
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. - PowerPoint PPT PresentationTRANSCRIPT
Monohybrid CrossesObjective1.Use the punnett square method to determine the possible genotypes of different monohybrid crosses2.Determine the phenotypic ratio of a monohybred cross.3.Explain how the process of meiosis and fertilization account for the transmission of inherited characteristics.
The transfer of characteristics (traits) from one generation to another
Your inherited characteristics (traits) are determined by the genes located on your chromosomes
Inheritance - Mendel
Dominant Trait
Description
# of students with Dominant
# of students with Recessive Ratio of
Dominant to
Recessive
Percentage with
Recessive traitsClass Class
Eye colour
Hair colour
Hairline
Freckles
Earlobe
Fingers
Ear rim
Thumb joint
Folded hands
Tongue rolling
Chin dimple
a short segment of a chromosome coding for one trait
Gene
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
Homologous chromosomes:
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
Phenotype
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
Genotype
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
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”
Dominant 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”
Recessive Trait/Allele
“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”
Homozygous 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”
Heterozygous Genotype
Summary (example eye colour)
Genotype Condition PhenotypeBB Homozygous
dominantBrown eyes
Bb Heterozygous Brown eyesbb Homozygous
recessiveBlue eyes
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 ___________.
Mendel’s Experiment
When individuals with different traits are crossed, the offspring (F1 generation) will express only the dominant trait.
Parents: Tall x ShortF1 (Offspring): Tall Tall Tall Tall
What must the genotype of the offspring be if the parents were pure breeding?
Principal of dominance
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)?
What happened next??
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.
Law of Segregation
Mendel crossed two purebred plants TT – tall plant Tt – short plant
Monohybrid cross – only one trait is being tested
Monohybrid Cross
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
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
Predicting genotype and phenotype possibilities using Punnett Squares
Egg
Spermc c
C Cc Cc
C Cc Cc
Punnett SquareAll of their children will have curly hair. They all have a different genotype from their parents.
Fig. 18.5
Fig. 18.6
Fig. 18.13
Fig. 18.14
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?
Codominance, Incomplete dominance and Dihybrid Crosses
Objectives:1. Explain what co dominance and
incomplete dominance is. Give an example of each.
2. Use a Punnett square to solve basic incomplete and co dominance crosses
3. Use a Punnett square to solve basic dihybrid crosses.
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
Codominance
When a red bull iscrossed with a white cow, the offspring willbe roan.
Incomplete DominanceWhen a red flower is crossedwith a red flower,the resulting offspringwill be pink.
Dihybrid Crosses A type of cross that involves two genes,
each consisting of non identical alleles.
RY RYryry
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).
What genotypes resulted?
What phenotypes resulted?
RY RYry RrYy RrYyry RrYy RrYy
Genotype: RrYy F1 - GenerationRatio 100%Phenotype: Round/YellowRatio 100%
RY Ry rY ryRYRyrYry
Crossing F1(RrYy) x F1(RrYy)
RY Ry rY ryRY RRYY RRYy RrYY RrYyRy RRYy RRyy RrYy RryyrY RrYY RrYy rrYY rrYyry RrYy Rryy rrYy rryy
What genotypes resulted?
What phenotypes resulted?
Genotype: RRYY RRYy Rryy RrYY RrYy rrYY rrYyrryy
Ratio: 1 2 2 2 4 1 2 1Phenotype: Round/Yellow Round/Green Wrinkled/Yellow Wrinkled/GreenRatio: 9 3 3 1
In the resulting F2 – Generation, the phenotypic ratio will always be 9:3:3:1In other words, the likelihood of obtaining each of the above phenotypes from crossing two hybrid pea plants is 9:3:3:1.
The inheritance of alleles for one trait ________ (does/does not) affect the inheritance of alleles for another trait.
Law of Independent Assortment