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Transcript of Unit 6
: a family tree that records and traces the occurrence of a trait in a family
squares = males
circles = females
colored shapes = individuals that show the trait
Parents are connected by horizontal lines, with their children beneath them in the order of birth.
example: Earlobes are either free or attached. This pedigree tracks the
occurrence of attached earlobes in three generations of a family.
example: red-green colorblindness
The half-filled circles represent carriers.
(This pedigree shows the appearance of colorblindness in four generations of a family.)
It is rare—but not impossible—for females to exhibit sex-linked (X-linked) traits.
To study the inheritance of a human trait:
collect information about a family's history for the trait
organize the information in a family tree
analyze this information by applying Mendel's concepts of dominant
and recessive alleles and his principle of segregation
A pea has gene loci for shape and color on different chromosomes. After this dihybrid pea matures into a plant, its chromosomes shuffle during meiosis, producing four possible genotypes in its gametes.
Meiosis and Mendel
Chapters 6, 7, & 9
Extending Mendelian Genetics
Mendel and Heredity
Traits and Probability
Meiosis and Genetic Variation
Complex Patterns of Inheritance
Gene Linkage and Mapping
Human Genetics and Pedigrees
Genetic Screening and Gene Therapy
Chromosomes and Phenotype
Frontiers of Biotechnology
: a distinguishing characteristic that is inherited
(example: a plant's trait of having red flowers, or yellow flowers, etc.)
: the study of biological inheritance patterns and variation
: an Austrian monk who found the basic rules of inheritance through a series of experiments
(He showed that traits are inherited as discrete units!)
: hypothesis in 1800s to explain how offspring inherit traits from both parents
flowered plants could produce an
BUT this was FALSE!
parental plants (P generation)
the gene for white flowers did not disappear in the F1 plants
notice: NOT a blend of purple and white
hybrid offspring (F1 generation)
Traits are inherited as discrete units.
Organisms inherit two copies of each
gene, one from each parent.
The two copies segregate during
: the process Mendel used to cross two true-breeding plants with two contrasting traits
cross seen above:
(Mendel experimented with these seven pea plant characters.)
pea plants - meaning when they self-fertilized, the offspring are identical to the parent pea plant.
pea plant makes
pea plant offspring)
Mendel bred pea plants and recorded inheritance patterns in the offspring for 7 years.
(Your book refers to these as
Traits, Genes, and Alleles
: the offspring of two different true-breeding varieties
Prior To Mendel
Mendel crossed the P generation to produce the F1 generation.
Notice the results:
F1 generation: all plants had
F2 generation: some plants had
flowers and some had
Mendel concluded that the trait for the white flower had not disappeared - it had been hidden.
Mendel performed many crosses to help him observe inheritance patterns.
Mendel's Three Conclusions
( and make up Mendel's Law of Segregation.)
: a piece of DNA that directs a cell to make a certain protein
Each gene has a
, a specific location on a pair of homologous chromosomes.
(Recall this from Unit 5?!)
: alternative forms of genes
(example: allele for blue eyes (
) vs. allele for brown eyes (
Each parent donates one allele for every gene.
: an allele in a heterozygous individual that appears to affect the trait
(example: allele for brown eyes (
: an allele in a heterozygous individual that does not appear to affect the trait
(example: allele for blue eyes (
: two alleles that are the same at a specific locus
: two alleles that are different at a specific locus
Genotype and Phenotype
: genetic makeup or combination of alleles (what the DNA says)
: an observable trait (what is physically seen)
: a grid system for predicting all possible genotypes resulting from a cross - it gives the ratio of possible genotypes and phenotypes of offspring
: examine the inheritance of only one specific trait
: examines the inheritance of two traits
Mendel's dihybrid crosses led to his second law, the
Law of Independent Assortment
This law states that allele pairs separate
of each other during meiosis.
: a cross between an organism with an unknown genotype and an organism with the recessive phenotype
: the likelihood that something will happen
Additional Videos on Heredity
all heterozygous, all dominant
More Examples of
1 homozygous dominant
1 homozygous recessive
1 homozygous recessive
All peas had the dominant phenotype: round and
This produced four phenotypes of peas.
Punnett square prediction:
a phenotypic ratio of 9 : 3 : 3 : 1
example: R can end up with either
, and r can end up with either
all offspring would be
: the ratio of the genotypes
: the ratio of the phenotypes
Sexual reproduction creates unique combination of genes.
independent assortment of chromosomes in meiosis
random fertilization of gametes
Unique phenotypes may give a reproductive advantage to some organisms.
: the exchange of chromosome segments between homologous chromosomes; occurs during the first part of meiosis I; results in new combinations of genes
Chromosomes contain many genes. The farther apart two genes are located on a chromosome, the more likely they are to be separated by crossing over.
: when genes located close together on a chromosome tend to be inherited together
Crossing over during Meiosis increases genetic diversity!
AKA - you don't know which gamete will be used to make the zygote!
(Think back to Mendel's law...)
Mendel's Law of Independent Assortment
Crossing over can recombine gene loci on homologous chromosomes. This is unlikely when the genes are very close together (Scenario 1). A crossover is more likely to recombine the alleles when the genes are far apart (Scenario 2).
are not linked to
because they are so far apart. Crossing over is likely to occur in the space between genes
, thereby separating
are referred to as linked because they would likely be inherited together.
are referred to as linked because they would likely be inherited together.
: a gene located on any of the autosomal chromosomes (chromosomes 1-22 in humans)
Two copies of each autosomal gene affect phenotype.
Mendel’s rules of inheritance apply to autosomal genetic disorders.
: a heterozygote for a recessive disorder
Disorders caused by dominant alleles are uncommon.
: genes on sex chromosomes
(chromosome 23 in humans)
Y chromosome genes in mammals are responsible for male characteristics.
X chromosome genes in mammals affect many traits.
male mammals: XY genotype
All of a male’s sex-linked genes are expressed.
Males have no second copies of sex-linked genes.
female mammals: XX genotype
Expression of sex-linked genes is similar to autosomal genes in females.
X chromosome inactivation randomly “turns off” one X chromosome.
Male and Female Genotypes
example: fruit flies
allele for red eyes = dominant
allele for white eyes = recessive
rare to find a female with white eyes.
Sex-linked disorders are disorders that are inherited as
sex-linked (X-linked) recessive traits.
(the same way as the white-eye trait in fruit flies)
examples: red-green color blindness and hemophilia
(a disease in which blood fails to clot normally)
These are more common in men.
(If a human male inherits the sex-linked recessive allele from his mother, the allele will be expressed - whereas females must inherit two alleles to exhibit the trait!)
This inheritance pattern is located only on the X chromosome. There is no corresponding eye color locus on the Y.
females (XX) carry two copies for eye color
males (XY) carry only one for eye color
(SO - a female will have white eyes only if she has the white-eye allele on both her X chromosomes, but males will only need one allele on their one X chromosome.)
Why might this be?
For some characters of organisms,
allele is dominant.
: pattern of inheritance where the heterozygous phenotype is
between the two homozygous phenotypes
Codominant alleles will both be completely expressed.
Codominant alleles are neither dominant nor recessive.
The ABO blood types result from codominant alleles.
: when two or more genes affect a single character - the variation in phenotypes can become even greater
example: height, eye color, and skin color (in humans)
An individual's phenotype depends on environment as well as on genes.
(example: a tree's genotype does not change, yet the tree's leaves vary in size, shape, and greenness from year to year (depending on exposure to wind and sunlight))
For humans, many phenotypes vary due to environmental factors.
Many characters of organisms have more complicated inheritance patterns than those studied by Mendel.
This inheritance pattern does
support the blending hypothesis.
(This is because the parent phenotypes can reappear in the F2 generation.)
example: red and white parents produce F1 hybrid offspring that are pink - neither the red nor white allele is dominant
three alleles for blood type in the human population: A, B, O
(But note that any one person has only two alleles for blood type.)
result in 6 genotypes & 4 phenotypes
Alleles IA and IB are codominant.
Allele i is recessive.
The individual's phenotype is not incomplete, but rather shows the
separate traits of both alleles.
On the other hand - some phenotype with little or no influence from the environment.
example: human blood type
In summary, the product of a genotype is generally not a single, rigidly defined phenotype, but a range of possibilities influenced by the environment.
Want more info on blood types?
Click the link: http://www.redcrossblood.org/learn-about-blood/blood-types
How to Study
Most human genetic disorders are recessive.
: an individual who has one copy of the allele for a recessive disorder and does not exhibit symptoms
example: a particular form of deafness is inherited as a recessive trait.
Types of Disorders
A smaller number of human disorders are inherited as dominant traits.
example: achondroplasia (a form of dwarfism - "little people")
(About 1 out of 25,000 people has achondroplasia - all individuals with this disorder are heterozygous. More than 99.99% of the population is homozygous for the normal, recessive allele so it clear that dominant alleles are not necessarily more plentiful than recessive alleles in a population.)
Disorders Inherited as Dominant Traits
Females can carry sex-linked genetic disorders.
Males (XY) express all of their sex linked genes.
Most sex-linked alleles are located on the X chromosome. A male only receives such sex-linked alleles from his mother. (The homologous Y chromosome is always inherited from the father.)
Therefore, if the phenotype is more common in males,
the gene is likely sex-linked.
Disorders Inherited as Recessive Traits
disorders inherited as recessive traits
disorders inherited as dominant traits
sex-linked disorders (both dominant and recessive)
dad & mom
How do you figure out genotypes of each?
determine the pattern in which the trait occurs
(example: Notice that the first-born daughter in the third generation has attached lobes, although both of her parents have free earlobes. - so the attached-earlobe trait must be recessive. If the trait for attached earlobes were dominant, then at least one of her parents would have attached earlobes.)
The genotypes of most family members can be determined.
Concept 7.3 covers much of the same information covered in Concept 6.6.
Students are responsible for the material in Concept 6.6 but may refer to 7.3 if desired for a more in-depth look at crossing over and genetic linkage.
: can detect genetic disorders; involves the testing of DNA
determines risk of having or passing on a genetic disorder
used to detect specific genes or proteins
can detect some genes related to an increased risk of cancer
can detect some genes known to cause genetic disorders
: the replacement of faulty genes
Gene therapy replaces defective or missing genes, or adds new genes, to treat a disease.
genetically engineered viruses used to “infect” a patient’s cells
insert gene to stimulate immune system to attack cancer cells
insert “suicide” genes into cancer cells that activate a drug
Gene Therapy Techniques
inserting gene into correct cells
controlling gene expression
determining effect on other genes
Gene Therapy Challenges
: two chromosomes of each matching pair (one chromosome in the pair comes from the mother, one comes from the father)
Meiosis consists of two distinct parts—
, homologous chromosomes, each composed of two sister chromatids, are separated from one another.
, sister chromatids are separated much as they are in mitosis. However, the resulting cells are haploid rather than diploid.
: a type of cell division that produces four cells, each with half the number of chromosomes as the parent cell
(In animals, meiosis occurs in the sex organs—the testes in males and the ovaries in females.)
: cells that contain two copies of every chromosome
example: body cells
: the total number of chromosomes in the cell (46 in humans)
: a cell with a single copy of every chromosome
example: gametes (sperm & egg cells)
new offspring cells, each with one set of chromosomes—thus
half the number of chromosomes
as the parent cell.
offspring cells, each with the
same number of chromosomes
as the parent cell.)
Meiosis produces four haploid cells, rather than two diploid cells. Also, meiosis involves two divisions, whereas mitosis involves only one.
This cell that has two homologous pairs of chromosomes
(2 from father (blue and green), 2 from mother (pink and purple)).
In each haploid daughter cell, a spindle forms, attaches to the centromeres, and moves the individual chromosomes to the middle of the cell.
Stage 1b (Prophase 2)
The process of meiosis is completed, producing
four haploid daughter cells
as a final result.
provides for growth and repair
produces daughter cells that are genetically identical to the parent cell
involves one division of the genetic material in the nucleus (usually accompanied by cytokinesis, producing two diploid cells)
produces gametes, which allows for sexual reproduction
yields haploid daughter cells with only one set of homologous chromosomes
involves two nuclear divisions, yielding four haploid cells
duplicated homologous chromosomes form tetrads
the chromosome number in each of the two daughter cells is haploid
Both Mitosis and Meiosis
chromosomes duplicate only once, in the preceding Interphase
Meiosis II is basically identical to mitosis. (The sister chromatids separate, and each cell divides in two.)
both make it possible for cells to inherit genetic information in the form of chromosome copies
Review: Comparison of Mitosis and Meiosis
Humans have 23 homologous pairs of chromosomes.
(So you have 46 chromosomes in total.)
: chromosome pairs 1-22 in humans
: determines the person's sex; the 23rd pair of chromosomes; occurs in two forms, called X and Y
Male mammals = XY
Female mammals = XX
Cells from males and females of the SAME species have the SAME number and types of chromosomes.
One from Dad
One from Mom
haploid sperm cell + haploid egg cell diploid zygote
: has two homologous sets of chromosomes, one set from each parent; develops into a sexually mature adult with trillions of cells produced by mitosis
: body cells
: sex cells
(example: egg and sperm cells)
Just as in mitosis, the cell duplicates its DNA. Just before meiosis, chromosomes condense and sister chromatids can be seen.
The tetrads move to the middle of the cell and line up across the spindle.
Stage 2A (Metaphase 1)
Proteins cause the homologous chromosomes to actually stick together along their length - the paired chromosomes, now consisting of four chromatids, are referred to as
. The tetrads attach to the spindle.
Stage 1A (Prophase 1)
Homologous chromosomes separate as they migrate to opposite poles of the spindle. (Sister chromatids migrate together.)
Stage 3A (Anaphase 1)
Stage 4A (Telophase 1) and Cytokinesis
Stage 4: The chromosomes arrive at the poles. Each pole now has a haploid daughter nucleus because it has only one set of chromosomes, even though each chromosome consists of two sister chromatids.
Cytokinesis forms two haploid daughter cells. The chromosomes in each daughter cell are still duplicated.
Stage 2b (Metaphase 2)
Stage 3b (Anaphase 2)
Stage 4b (Telophase 2) and Cytokinesis
The chromosomes line up in the middle of the cell with spindle microtubules attached to each sister chromatid.
The sister chromatids separate and move to opposite poles.
The chromatids, now considered individual chromosomes, arrive at the poles.
Cytokinesis splits the cells one more time.
Chromosomes and Meiosis
Process of Meiosis
: a display of all of the chromosomes of an individual (46 in humans)
The Human Life Cycle
Each gamete made from meiosis can have a large variety of possibilities for genetic variation.
(Remember, human gametes have 23 chromosomes, so the possibilities are numerous! This is why you and your siblings are not identical.)