Unit 8: Genetics & Heredity
Unit 9: Human Genetic Disorders
Ch. 26: Inheritance of Traits
& Ch. 27: Human Genetics
200
Unit 8: Genetics & Heredity
• What is genetics?
– the study of heredity
• passing of traits from parents to offspring
Chromosomes in Cells
• Remember…
– body cells are diploid
• 2 of each
chromosome
– 1 from mom & 1 from
dad
– gametes (sperm &
eggs) are haploid
• 1 of each
chromosome
– Why?
» So zygote gets right #
of chromosomes…
Genes
• Why is your
combination of genes
unique?
– Chance… don’t know
which sperm will fertilize
which egg…
• get ½ of your
chromosomes from mom
& ½ from dad
• meiosis (formation of
gametes)
– crossing-over during
prophase 1
– alignment of chromosomes
during metaphase 1
Genes & Alleles
• What is a “gene”?
– section of chromosome that
codes for a specific protein
• & determines a specific trait (ex. hair
color, eye color, ear shape, etc.)
– genes are paired on
homologous chromosomes
• different (or alternate) forms of
genes for the same trait are
called “alleles”
Dominant & Recessive Alleles
• Each parent contributes 1 allele (form of gene)
for trait & can be dominant or recessive
– What is a dominant allele?
• allele that prevents expression of (“masks”/“hides”)
recessive trait
– What is a recessive allele?
• allele whose trait can be seen only when the organism is
pure (homozygous) for that trait
Dominant & Recessive Alleles
• How are alleles
represented?
– with letters
• usually the first letter
of the dominant trait
– If the same letter is
used for dominant &
recessive, how do we
know which allele is
which?
» CAPITAL =
DOMINANT
» lowercase =
recessive
Allele Combinations
• What does “homozygous” mean?
– both alleles are the same
• homozygous (pure) dominant (ex. AA)
• homozygous (pure) recessive (ex. aa)
• What does “heterozygous” mean?
– both alleles are different
• heterozygous (hybrid) (ex. Aa)
Genotype vs. Phenotype
• What is “genotype”?
– organism’s actual genetic
“code”/make-up (alleles)
• What does the genotype do?
– codes for protein that causes
trait (phenotype)
• How do we represent an
organism’s genotype?
– 2 letters (one for each allele)
• one from mom & one from dad
– ex. PP, Pp, pp
Genotype vs. Phenotype
• What is “phenotype”?
– the outward (physical) expression
of the genotype (trait we “see”)
• What actually causes the
“phenotype” (trait) we see?
– the protein that is produced
(due to the organism’s genotype
“code”/alleles)
• How do we represent an
organism’s phenotype?
– usually an adjective
• ex. purple, white, tall, short, etc.
Genotype is Expressed as a Phenotype
• Ex. Let P = purple & p = white
– homozygous (pure) dominant
• genotype PP
• phenotype = purple
– homozygous (pure) recessive
• genotype pp
• phenotype = white
– heterozygous (hybrid)
• genotype Pp
• phenotype = purple
– dominant trait “masks/hides”
recessive trait
PP
Pp
pp
Predicting Traits in Offspring
• What are Punnett Squares?
– a way to predict the results of crosses (mating)
• letters outside represent possible alleles in gametes of
each parent
– top = one parent & side = other parent
• letters inside boxes represent possible allele
combinations (genotypes) in offspring (& phenotypes)
– can be used to determine probability and ratios
BB
Bb
Making a Punnett Square
• Parents are Tt & tt genotypes…
– So… Tt x tt is our cross (mating)
Passing Traits to Offspring & Probability
• What is probability?
– chance an event will occur
– What is the chance of
getting heads? tails?
• ½
– If you flip two coins, of
getting 2 heads? 2 tails?
• ½ x ½ = 1/4
– What is the chance of a
couple having a boy? a girl?
• 1/2
– of having five girls?
• ½ x ½ x ½ x ½ x ½ = 1/32
– or ( ½ )5 = 1/32
Passing Traits to Offspring & Ratios
• What is a
“genotypic ratio”?
– probable ratio of
genotypes (alleles)
in offspring of a
given cross
• Ex. If cross Pp & Pp
– 1PP : 2Pp : 1 pp
Passing Traits to Offspring & Ratios
• What is a
“phenotypic ratio”?
– probable ratio of
phenotypes (traits)
in offspring of a
given cross
– resulting from the
genotypes of the
offspring
• Ex. If cross Pp & Pp
• 3 purple : 1 white
Passing Traits to Offspring & Ratios
• What is an
“expected ratio”?
– ratio we expect to get
based on probability
(Punnett Square)
• What is an
“observed ratio”?
– ratio we actually get
• Why would these be
different?
– fertilization is random
– some embryos die
during early stages
Gregor Mendel
• Father of Genetics
– 1822-1884
• studied garden pea
plants
– 7 different traits with
clearly different forms
• tried to determine how
they were passed from
parent to offspring
Mendel’s Experiments
• What happened when Mendel mated
a pure purple parent (PP) & a pure
white parent (pp)?
–all offspring had:
• purple phenotype
• heterozygous
(hybrid) genotype
–Pp
Mendel’s Experiments
• What happened when Mendel let the
heterozygous (hybrid) offspring from his
first experiment self-pollinate?
– So… Pp x Pp
• new offspring weren’t all purple…
Mendel’s Principle of Dominance
• What did Mendel notice
from his experiments?
– that one form dominates
over the other
• …dominant trait prevents
the expression of the
recessive trait
– What trait was dominant
in these plants?
» PUPRLE = dominant
– What trait was recessive?
» white = recessive
Dominant/Recessive is Not
Always the Method of Inheritance
• Traits are not always as clearly defined as
the 7 pea plant traits Mendel studied.
– examples of non-dominant/recessive
inheritance
• sex determination
• sex-linked traits
• codominance
• multiple alleles
Sex Determination
• How many chromosomes do
humans have (in body cells)?
– 46… 23 pairs
• pairs 1 – 22 = autosomes (body
chromosomes)
• 23rd pair determines gender = sex
chromosomes
– XX = female
– XY = male
• Which parent’s chromosomes
determines if the offspring will be
a boy or girl???? Why?
– Dad’s b/c he is the only one that
can give a Y; mom always gives X.
What is the
probability of
having a son?
A daughter?
Sex-linked Inheritance
• X & Y chromosomes not fully homologous.
Why?
– X is bigger & carries more genes
Sex-linked Inheritance
• How many alleles will a male have for
traits carried only on the X chromosome?
– 1 b/c only have 1 X chromosome (Y doesn’t
have allele)
• What is this called?
– X-linked or sex-linked
» Ex. eye color in fruit flies, hemophilia in humans,
colorblindness in humans
– X-linked traits & disorders are more common
in males. Why???
• b/c female has XX, more likely she will have a
copy of dominant allele… males = XY… can only
get dominant allele on X (& only have 1 X)
Sex-linked Inheritance
• How do we make predictions made using Punnett
squares for sex-linked traits?
– Consider the sex chromosome (X/Y) & allele for the trait
it carries (“exponent”) TOGETHER as a unit…
• ex. XG (= X w/ dominant allele), Xg (= X w/ recessive allele), Y (= Y
w/ NO allele)
• What if a female is heterozygous (XGXg)?
– she does not show the
trait/have the disorder,
but is a carrier
• & can pass gene
to offspring
XG
Xg
XG
XG XG
XG Xg
Y
XG Y
Xg Y
• Can a male be a carrier?
– No, b/c only has one X chromosome
w/ allele… so either has it or doesn’t
Sex-linked Inheritance
• Drosophila (fruit fly) eye color is sex-linked
– What are the sex, genotype, & phenotype of each
offspring? Are there any carriers for the white eye
gene?
• Left picture: 2 females with red eyes = XRXr (carrier white
eye gene) & 2 males with white eyes = XrY
• Right picture: female w/ red eyes = XRXR, female w/ red
eyes = XRXr (carrier white eye gene), male w/ red eyes =
XRY, & male w/ white eyes = XrY
XR
Y
XR
Xr
XR XR
XR Y
XR Xr
Xr Y
Multiple Alleles & Codominance
• What is meant by
multiple alleles?
– more than 2 different
forms of an allele exist
• but individual still has
just 2
– Ex. human blood types
• (3) multiple alleles
– A (IA)
– B (IB)
– o (i)
•How many possible
genotypes are there?
•How many phenotypes?
•Can you spot the blood
type that is the result of
codominance?
Multiple Alleles & Codominance
• What is meant by
codominance?
– both alleles are
“expressed” equally
– Ex. human blood types
also exhibit codominance
(as well as multiple alleles)
• A & B are codominant and
are “expressed” equally
– A = B (codominant)
– o (recessive)
» So… (A = B) > o
•How many possible
genotypes are there?
•How many phenotypes?
Unit 9: Human Genetic Disorders
• What causes genetic disorders?
– DNA mutation (usually recessive) or chromosome
abnormalities (in # or structure) that cause the
production of abnormal proteins
Human Genetic Disorders
• How can we group genetic disorders?
1. autosomal recessive disorders (*most genetic disorders)
• allele is recessive & found on a chromosome from pairs 1 – 22
(autosomes or body chromosomes)
– cystic fibrosis, sickle-cell anemia, Tay-Sachs disease
2. autosomal dominant disorders
• allele is dominant & found on a chromosome from pairs 1 – 22
(autosomes or body chromosomes)
– Huntington’s Disease
3. sex-linked disorders
• allele (which is usually recessive) is found on the 23rd pair of
chromosomes (sex chromosomes)… Usually on the X chromosome
– hemophilia, color blindness
4. chromosomal abnormality disorders
• result from errors in chromosome # or structure
– Down Syndrome (trisomy 21), Klinefelter’s Syndrome (XXY)
Autosomal Recessive Disorders
• What genotype(s) must a
person have to be affected?
– homozygous recessive (gg)
• cystic fibrosis
• sickle-cell anemia
• Tay-Sachs Disease
• Can someone be a carrier?
Why/why not?
– yes
• b/c if heterozygous (Gg), person
carries the gene, but isn’t affected
– due to having the “normal”
dominant gene
Autosomal Dominant Disorders
• What genotype(s) must a
person have to be affected?
– can be homozygous (GG) or
heterozygous (Gg) b/c allele is
dominant
• Huntington’s Disease
• Can someone be a carrier?
Why/why not?
– No
• b/c even if person is heterozygous
(Gg), person will have disorder
– due to dominant “disease” gene
blocking “normal” recessive gene
Sex-linked Disorders
• Remember from earlier… hemophilia is
X-linked & recessive
– What are the possible genotypes &
phenotypes? Can someone be a carrier?
•
•
•
•
•
XHXH = normal female
XHXh = carrier female (but not affected)
XhXh = female w/ hemophilia
XHY = normal male
XhY = male w/ hemophilia
– Why can’t a male be a carrier?
• b/c only has one X chromosome
w/ allele… so either has it or doesn’t
– Ex. mom = carrier & dad = normal:
• Make a Punnett square.
– genotypic ratio?
– phenotypic ratio?
1 XHXH: 1 XHXh: 0 XhXh: 1 XHY: 1 XhY
1 normal female: 1 carrier female : 0 female w/ hemophilia: 1 normal male:
1 hemophiliac male
Sex-linked Disorders
• Remember from earlier…
colorblindness is X-linked recessive
– What are the possible genotypes &
phenotypes? Can someone be a carrier?
•
•
•
•
•
XCXC = normal female
XCXc = carrier female (but not affected)
XcXc = colorblind female
XCY = normal male
XcY = colorblind male
Ishihara
test for
redgreen
colorblindness
– In this Punnett square, what are the
genotypes & phenotypes of the parents?
• father:
– genotype = XCY & phenotype = normal
• mother:
– genotype = XCXc & phenotype = carrier
Chromosomal Abnormalities in Number
• What causes an
abnormal
number of
chromosomes?
– non-disjunction
• failure of paired
chromosomes to
separate during
meiosis 1 or
meiosis 2
Disorders Due to Abnormal Chromosome #
• What is Down Syndrome (trisomy 21)?
– when person has 3 copies of chromosome # 21
• What is Klinefelter’s Syndrome?
– a sex-chromosome disorder in which males have extra copy
of X chromosome
• XXY (or 47, XXY b/c 47 total chromosomes)
• What causes Down Syndrome (trisomy 21) &
Klinefelter’s Syndrome?
– non-disjunction
• failure of paired chromosomes to separate during meiosis 1 or
meiosis 2
Chromosomal Abnormalities in Structure
• What is causes
structural
abnormalities in
chromosomes?
− pieces are
added, deleted,
inverted, or
translocated
Review & Animations
• Vocab interactive
– http://nortonbooks.com/college/biology/animations/ch10a02.htm
• Crosses
– http://www.sonefe.org/online-biyoloji-dersleri/grade-12/monohybridcross/
• Drag & drop genetics
– http://www.zerobio.com/drag_gr11/mono.htm
• Various
– http://www.abpischools.org.uk/page/modules/genome/dna4.cfm?coSiteN
avigation_allTopic=1
• Genetic disorders
– http://www.humanillnesses.com/original/Gas-Hep/Genetic-Diseases.html