Transcript mendel intro

Mendelian Genetics
1. Pick up the handouts.
Homework
 Worksheet due tomorrow.
 Heads up: There will be a bunch of
homework for this unit because you
will need to practice.
Objectives
 Understand the connection
between chromosome, gene,
base sequence, allele, and
genotype.
 Use allele conventions to write
genotypes.
 Connect dominance to
genotype and phenotype.
Review
 With your partner, talk and look up
definitions for these terms:
 Chromosome
 Gene
 Gamete
 Trait
Review
 Chromosome: A very long, tightly-
wound strand of DNA. Contains
thousands of genes.
 Gene: A segment of a chromosome
that codes for a protein.
 Gamete: A haploid sex cell (sperm
and egg).
 Trait: The physical characteristic
you get from the proteins made by
one or more genes.
Genes and Traits
 This unit is going to be about
INHERITANCE. The transmission of
traits across generations, how we
quantify traits, and how we can
predict what versions of genes you,
your relatives, and your offspring
must have based upon limited
information.
 (Our next unit will then be taking this
same skill and applying it to whole
populations.)
 Today, there will be several new
vocab words. Prepare yourselves!
Genes and Alleles
 Demo connection between:
 Chromosome
 Gene
 Base sequence
 Allele
Genes and Traits
 Our first new word, because saying
“version of a gene” every time is powerfully
tiresome.
 Allele: A version of a gene. For instance,
when we’re talking about the gene for hair
color, different alleles may be for black
hair, brown hair, blonde hair, and red hair.
 (Every gene has at least two alleles. Most of
them have more, but for the sake of simplicity,
we’ll usually talk about them in terms of just
two.)
 With your partner: Pick any two human
traits that could be coded for by a gene.
What could be different alleles of that
gene?
Genes and Traits
 The gene is the region of the
chromosome, the allele is the sequence of
bases in that region.
Genes and Traits
 There are two other words that I
introduced to you ages and ages ago,
does anybody recall what they mean?
 Genotype and Phenotype
Genes and Traits
 Genotype: Your combination of
alleles.
 Think “Genotype = Your type of
gene”
 Phenotype: Your trait or
combination of traits.
 Think “Phenotype = Your physical
type”
Genotypes and Phenotypes
 Take out the handout w/ the
three students’ chromosomes,
and the chart.
 These are the chromosome #3s
for three people.
 Why do they each have two
chromosome #3s?
 What observations do you have
about them?
Genotypes and Phenotypes
 When we’re talking about
genotypes, scientists represent the
alleles with letters.
 It’s annoying to write out 2000 base
pairs every time you want to mention
the allele, so we shorten it.
 We’ll usually use one letter for each
gene, like the letter R. Then, the
different alleles are represented by the
R being either upper-case R or lowercase r.
 What trait does the gene
represented by letters R code for?
 The other letters?
Genotypes and Phenotypes
 Compare these students’
tongue-rolling genotypes.
 Which of these students’
chromosomes have identical
base sequences in the tonguerolling gene region?
Genotypes and Phenotypes
 Both of Joaquin’s chromosomes have
a tongue-rolling gene with exactly the
same sequence of bases. So do both
of Darnell’s, though his sequence of
bases is different from Joaquin’s.
 Jer, on the other hand, has a tongue-
rolling gene with one allele on one
chromosome, and a different allele on
the other chromosome. His two
copies of that gene has different
sequences from each other.
 What does this mean in terms of the
proteins these students are making?
Genotypes and Phenotypes
 We express genotypes as that
individual’s two alleles written
together, like this:
 Joaquin has the genotype rr
 Darnell has the genotype RR
 Jer has the genotype Rr
 If the two alleles are different, we
always write the capital letter first.
Like writing a name.
Genotypes and Phenotypes
 Take a moment and fill out the
middle column for all three
students. Use the information
from their chromosomes to
correctly write their genotypes.
 Ignore the “yes and no” part for
now.
Genotypes
 Alleles aren’t just randomly given a
capital or lower-case letter. The
capital letter means something.
 Dominant alleles: If either of your
chromosomes has that allele, you will
have the dominant version of the trait.
Use capital letters.
 Recessive alleles: They will only give
you the recessive version of the trait if
you do NOT have the dominant allele
anywhere.
 Use lower-case letters.
Genotypes
 Dominant alleles are NOT more
common than recessive alleles,
and they are NOT “better” or
“stronger” than recessive
alleles.
 Dominant only refers to which one
will express itself if you happen to
have both. Nothing else.
Genotypes
 Look at the chromosomes.
 What kind of alleles do the three
students have for tongue-rolling?
 Which, if any, of them will have
the dominant version of the
tongue-rolling trait?
Genotypes
 Who remembers what the roots “homo-”
and “hetero-” mean in words?
Genotypes
 Who remembers what the roots
“homo-” and “hetero-” mean in
words?
 Latin/Greek Roots
 Homo: Same
Homogenous, homosexuality,
homogenized, homeostasis
 Hetero: Different
Heterogenous, heterosexuality,
heterodox
Genotypes
 Heterozygous: A genotype with two
different alleles. Tt, Yy, Uu, Kk…
 Notice: hetero + zygote
 Homozygous: A genotype with two
of the same allele.
 Homozygous dominant: Both alleles
are dominant. TT, YY, UU, KK…
 Homozygous recessive: Both alleles
are recessive. tt, yy, uu, kk…
Genotypes
 Look again at your chromosomes.
 Find every instance where someone is
homozygous dominant.
Genotypes
 Look again at your chromosomes.
 Find every instance where someone is
homozygous recessive.
Genotypes
 Look again at your chromosomes.
 Find every instance where someone is
heterozygous.
Genotypes and Phenotypes
 Homozygous dominant and
heterozygous people will
always have the dominant
version of that trait.
 Only homozygous recessive
people will have the recessive
version.
 Discuss with partner in just 15
seconds: why?
Phenotypes
 Let’s finish by analyzing
phenotypes for Joaquin, Darnell,
and Jer.
 Where it says “Yes,” it means that
students has the listed version of the
trait.
 Where it says “No,” it means that
student doesn’t have that version of
the trait.
 How can we figure out what is the
dominant phenotype and what is the
recessive phenotype?
Genotypes and Phenotypes
 Your homework gives you
practice with genotypes and
phenotypes, heterozygous
and homozygous, dominant
and recessive. Some points
to clarify:
 Why does everyone have two
alleles for every gene?
Genotypes and Phenotypes
 Your homework gives you practice
with genotypes and phenotypes,
heterozygous and homozygous,
dominant and recessive. Some
points to clarify:
 Why does everyone have two alleles
for every gene? Because… Cells are
diploid, so you have 2 chromosomes
of every kind, and each chromosome
carries 1 allele for a gene.
Genotypes and Phenotypes
 Your homework gives you practice
with genotypes and phenotypes,
heterozygous and homozygous,
dominant and recessive. Some
points to clarify:
 For each gene, do you think you got
both alleles from Mom? Both alleles
from Dad? One allele from each?
Genotypes and Phenotypes
 Your homework gives you practice with
genotypes and phenotypes,
heterozygous and homozygous,
dominant and recessive. Some points to
clarify:
 For each gene, you received one allele
from Mom, and one allele from Dad. This
is because you came from the zygote
created from the fusion of their two
haploid gametes. Each gamete contained
one copy of the chromosome.
 Can we tell just by looking at this piece of
paper which chromosome Joaquin,
Darnell, or Jer got from Mom and which
he got from Dad?
Genotypes and Phenotypes
 Your homework gives you practice with
genotypes and phenotypes, heterozygous
and homozygous, dominant and
recessive. Some points to clarify:
 Dominant alleles don’t just come from one
kind of parent, they have an equal chance
of coming from Mom or from Dad. The
only way to tell which allele came from
whom is by looking at the parents’
genotypes as well as yours.
Genotypes and Phenotypes
 Use this time to work on your
homework. Get REALLY
GOOD at understanding allele
vs gene, homozygous vs
heterozygous, dominant vs
recessive. If you are
confused, ask! HELP ME
HELP YOU. If you don’t get
this now, you’re so going to be
toast for the next two weeks.
Sad face. :(
Mendelian Genetics
 Which is a genotype and which is a
phenotype?
 Bb
Five-fingered hands
LL
Diabetes
 Which alleles are dominant vs
recessive?
Y
i
K
b
d
 Which are heterozygous, homozygous
dominant, homozygous recessive?
 Uu
YY
hh
Rr
Mendelian Genetics
 Find these definitions in your notes from
yesterday:
 Allele
 Genotype
 Phenotype