MENDELS LAWS, MONOHYBRID AND DIHYBRID CROSSES Mrs. Stewart
Honors Biology
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Bell work Describe the relationship between genotype and
phenotype
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STANDARDS CLE 3210.4.1Investigate how genetic information is
encoded in nucleic acids. CLE 3210.4.3 Predict the outcome of
monohybrid and dihybrid crosses.
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OBJECTIVES Analyze the law of segregation Create a punnett
square using the genotypes of parents Predict the outcome and
probability of monohybrid crosses
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DECIDE WITH YOUR PARTNER
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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
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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 persons 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
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DOGS Tell your CAT how to differentiate between a
characteristic and a trait
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HOW DO WE INHERIT TRAITS FROM OUR PARENTS? Remember
Meiosis?
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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
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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
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CATS Tell your DOG how we inherit traits from our parents
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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).
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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
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LAW OF SEGREGATION Homologous chromosomes separate during the
formation of gametes
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DOGS Explain the law of segregation to your CAT
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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.
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CATS Explain the law of independent assortment to your DOG
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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.
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THINK PAIR SHARE CATS AND DOGS How can we predict the
inheritance of traits? Punnett Squares
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PUNNETT SQUARES? Punnett Squares use genotypes to predict
inheritance Punnett Squares show the law of segregation in action
MomMom Dad What do the letters on the outside of the punnett square
represent? What do each of the squares inside the punnett square
represent?
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DOES IT MATTER WHICH SIDE YOU PUT THE PARENTS GENOTYPE ON? bb
BBb B No, the results are the same.
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MONOHYBRID CROSS Monohybrid = a cross between two organisms
that predicts the inheritance pattern/probability of only one
characteristic at a time
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PRACTICE TOGETHER 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 BbBb B B
BB Bb 0
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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
%
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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
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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 Bb bb BBBB
TEST CROSS Guinea 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 genotype
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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? Bb bb
BBBB Bb bb b BbBb
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THREE TYPES OF DOMINANT RELATIONSHIPS FOR ALLELES Complete
dominance Incomplete dominance Codominance
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COMPLETE DOMINANCE Standard dominant vs recessive
relationship
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INCOMPLETE DOMINANCE One is not completely dominant over the
other Instead, heterozygotes will have a blending of the dominant
and recessive traits
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CODOMINANCE Both alleles for a gene are expressed in the
heterozygotes simultaneously Neither allele is dominant or
recessive, nor do they blend together
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DIHYBRID CROSSES A cross in which TWO characteristics are
tracked
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USE F.O.I.L. METHOD TO DETERMINE GAMETES
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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 BLO Bl I bLL bl You do the next
one on your worksheet
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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