Transcript Chapter 5 - St. Clair Schools

Chapter 5
Heredity
Section 5.1 Mendel & Peas
Heredity
 Passing of genetic traits from
parent to offspring
How Traits are Inherited
 Genes made up of DNA
 Genes found on
chromosomes
 Genes control all traits
 Pairs of genes separate
when chromosomes
separate
 Known as Father of Genetics
 Discovered the principles of heredity by
studying pea plants
 Noticed that traits can skip a generations
 Chose peas because they grow quickly, many different kinds, & are
self-pollinating (has both male & female reproductive parts – pollen
from one flower can fertilize same flower)
 Mendel able to grow true breeding plants (all offspring have same
trait as parents)
 Peas can also cross-pollinate (pollen from one plant fertilizes the egg
of another plant)
 Pollen carried by insects, wind, & other animals
 Mendel studied one characteristic at a time (seed
shape, plant height, flower color)
 Another example: hair color in humans is a
characteristic & different forms, like red, brown, or
blonde, are traits
Dominant vs. Recessive
 Dominant traits are always expressed
(observed) in first generations when parents
w/different traits are bred
 Recessive traits are masked or hidden in first
generations but reappear in the second
generation when parents w/different traits are
bred
 Mendel decided to
figure out ratio of
dominant to recessive
traits
 Ratio is relationship
between two different
numbers (often shown
as a fraction) Example:
3:1
Section 5.2 - Traits & Inheritance
Genes:
 Set of instructions for an inherited trait
Alleles:
 Different forms of a gene (ex. all have a
gene for eye color but each have different
alleles such as blue, brown, green)
Dominant
 trait that is always expressed
 shown with a capital letter (B)
 can be homozygous or heterozygous
Recessive
 trait that is not expressed when dominant
allele is present; dominant allele mask or
covers it up.
 shown with a lowercase letter (b)
 must be homozygous for recessive trait
to be expressed
Purebred:
 Both alleles are the same (ex. BB or bb)
 Can be homozygous dominant (BB) or
homozygous recessive (bb)
 Means the same as Homozygous
Hybrid:
 Both alleles are different (ex. Bb)
 Means the same as Heterozygous
Phenotype:
 Physical appearance or what you see
 Example: such as brown eyes vs. blue
eyes.
Genotype:
 Genetic makeup of an organism
 Set of alleles can be - Bb, BB, or bb
Punnett Square
 diagram that shows the expected offspring of 2
parents (shows all the possibilities)
 Capital letter = dominant allele
 Small letter = recessive allele
 Letters of one parent written along top of
square and letters of other parent written along
the left side of the square
 Cannot always figure out genotype by looking
at the phenotype
Example: First Generation
 Alleles from homozygous tall parent
 Alleles from homozygous short parent
 Genotypes of offspring:
 Phenotype of offspring:
Second Generation
 Alleles from heterozygous tall parents
 Genotypes of offspring:
 Phenotype of offspring:
Probability
 Predicts the chance that something will
happen (ex. coin toss 1 out of 2 or a 50%
chance)
 Think lottery, weather forecasting.
Incomplete Dominance
 Phenotype is intermediate (in
between) to the 2 homozygous
parents
 Neither allele for color was dominant –
colors blended to make new color
 Example: four o’clock flowers have
alleles for red and white flowers
Red x White = Pink
One gene-Many Traits
 Sometimes one gene can influence more
than 1 trait
 Single trait that is produced by a
combination of many genes
 Example: In tigers the gene for fur color
also carries the gene for eye color
white tiger - -> white fur caused by
single gene but this gene also
influences other traits like eye color
(tiger has blue eyes)
Section 5.3 Meiosis
Asexual Reproduction
 Only one parent cell is needed (these cells have 46
chromosomes)
 Structures are copied and then parent cell divides
making two identical cells (this is Mitosis & occurs in
body cells)
 Daughter cells are identical (46 chromosomes)
 Bacteria, single celled organisms
Sexual Reproduction
 Two parent cells join together to form an
offspring that are different from both
parents
 Two sex cells join – one from each parent
(each sex cell has 23 chromosomes)
Sexual Reproduction
 Human sex cells join (1 egg, 1
sperm)
 Human sex cells have 23
chromosomes
 Process is known as Meiosis
Meiosis
 Process in cell division in which
number of chromosomes are
reduced to half the original number
 Cells go through cell division 2 times
Meiosis
 Sex cell in the female is the egg (23
chromosomes)
 Sex cell in the male is the sperm (23
chromosomes)
 Sperm and egg join to make offspring
with 46 chromosomes (23 + 23 = 46)
Homologous
Chromosomes
Chromosomes
that carry the
same set of
genes
(like a pair of
shoes)
Steps of Meiosis
1. Starts with parent cell (46 chromosomes)
2. Interphase-chromosomes copy (sister
chromatids)
3. Prophase I -chromosomes become visible;
nuclear membrane disappears
4. Metaphase I -homologous chromosomes
pair up, pairs of chromosomes line up at the
center of the cell
Steps of Meiosis
5. Anaphase I - homologous pairs start to
separate
6. Telophase I -homologous pairs move to
opposite ends of cell; nuclear membrane
reforms
7. Now you have 2 cells
Steps of Meiosis
8. Prophase II - chromosomes become visible;
nuclear membrane disappears
9. Metaphase II - chromosomes line up at
equator of cell
10. Anaphase II - sister chromatids of
chromosomes separate
Steps of Meiosis
11. Telophase II - chromatids move to opposite
ends of the cell; nuclear envelope reforms
12. Cytokinesis - cytoplasm divides
13. END with 4 sex cells- each has ½ number of
chromosomes (23)
Sex Chromosomes
 X and Y chromosomes
 Male has XY
 Female has XX
 All eggs have an X chromosome
 Sperm will EITHER have an X OR a Y chromosome
 When they join determines if offspring is male or
female
Sex Linked Disorders
 Some inherited conditions are carried on a
sex chromosome
 Females get 2 X chromosomes-if one is
unhealthy they have a back up
 Males only get one X chromosome-if it is
unhealthy they will have a disorder
Color-Blindness




Sex linked disorder
Trouble distinguishing red from green
Males are color-blind more often than females
If mom has an allele for colorblindness and
passes it on to a son he will be color blind
 Females must have a dad who is color-blind
and a mom who carries the color-blind allele to
be color-blind
Hemophilia
 Sex linked disorder
 Blood does not clot properly
 People with this disorder bleed a lot
from small cuts
 Can be fatal
Sickle-cell anemia
(recessive disorder)
 Homozygous recessive disorder
 Blood cells are sickle shaped
(like a C) instead of normal disc shaped
 Cannot deliver enough oxygen
 Blood cells get stuck together
 Most people with disorder die as children
Cystic fibrosis (recessive
disorder)
 Homozygous recessive
 Normally thin fluid lubricates lungs but people
with CF have Thick mucus clogs lungs
 Makes it hard to breathe causes lung damage
 1 in 20 people carries a recessive allele for this
disorder
Pedigree Charts
 Family tree that traces a trait in a family
 Purebred dogs might have a pedigree chart
 Used in tracking disease & genetic counseling
Pedigree Charts
Square = male
Circle = female
Shaded in = has trait
Half shaded = carrier
Not shaded = normal
Selective Breeding
 Organisms with desirable characteristics
are mated
 Humans have been selectively breeding
for thousands of years (10,000)
 Started after the last ice age
 Examples: chickens that produce larger
eggs, dog breeding, thorn-less roses