1. Human Heredity
Chapter 14-1, 14-2, 14-3

2. Human Chromosomes Humans have 46 Chromosomes- 23 pairs
Karyotype – a picture of arranged chromosomes. The picture is created by photographing cells during mitosis and then “cutting and pasting” the chromosomes from the photograph into groups or pairs.

3. Human Chromosomes
Two of the 46 human chromosomes are known as sex chromosomes, because they determine an individual's sex.
Females have two copies of an X chromosome (XX).
Males have one X chromosome and one Y chromosome(XY).
The remaining 44 chromosomes are known as autosomal chromosomes, or autosomes.

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Human Chromosomes
All human egg cells carry a single X chromosome (23,X).
Half of all sperm cells carry an X chromosome (23,X) and half carry a Y chromosome (23,Y).
About half of the zygotes will be 46,XX (female) and half will be 46,XY (male).
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Human Chromosomes
Males and females are born in a roughly 50 : 50 ratio because of the way in which sex chromosomes segregate during meiosis.
In humans, egg cells contain a single X chromosome. Sperm cells contain either one X chromosome or one Y chromosome. In a population, approximately half of the zygotes are XX (female) and half are XY (male).
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6. Sex-Linked Genes
Genes located on the X or Y chromosomes are called sex-linked genes.
There are more than 100 disorders that have been mapped on the X chromosome. Many of these are recessive traits. Because men only have one X chromosome, these disorders are more common in men.

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Sex-Linked Genes
X Chromosome
Duchenne muscular dystrophy
The Y chromosome is much smaller than the X chromosome and appears to contain only a few genes.
Melanoma
X-inactivation center
Colorblindness
Hemophilia
X-linked severe combined immunodeficiency (SCID)
Genes on X and Y chromosomes, such as those shown in the diagrams, are called sex-linked genes.
Y Chromosome
Testis-determining
factor
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8. Sex-Linked Genes Hemophilia Color Blindness
In hemophilia, a protein necessary for normal blood clotting is missing.
Hemophiliacs can bleed to death from cuts and may suffer internal bleeding if bruised.
Color Blindness
The gene for color blindness is located on the X chromosome

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Sex-Linked Genes
Possible Inheritance of Colorblindness Allele
X-linked alleles are always expressed in males, because males have only one X chromosome. Males who receive the recessive Xc allele all have colorblindness. Females, however, will have colorblindness only if they receive two Xc alleles.
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Colorblindness
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11. Human Traits
One can study how a trait is passed from one generation to the next.
A pedigree chart, which shows the relationship within a family of a single trait.
One can infer the genotypes of family members based on when and how a trait is passed from one generation to the next.
A pedigree is not useful when dealing with polygenic traits or some genes that are influenced by nature.

13. Human Traits A square represents A circle represents
a male.
A circle represents
a female.
A vertical line and a bracket connect the parents to their children.
A horizontal line connecting a male and a female represents a marriage.
A shaded circle or square indicates that a person expresses the trait.
A circle or square that is not shaded indicates that a person does not express the trait.
This drawing shows what the symbols in a pedigree represent.
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15. Recessive Allele Disorders
These are genetic disorders that are only expressed when the individual has both the recessive alleles.
Examples
Albinism – Lack of pigment
Cystic fibrosis – excess mucus in the lungs
PKU – unable to break down phenylalanine, can cause many problems including mental retardation.
Tay-Sachs disease – lipid accumulation in brain cells, death in early childhood.

16. Dominate Allele Disorders
These are disorders that occur based on a dominate allele. Therefore it will be expressed even if you only have a single allele.
Example
Dwarfism – achondroplaia
Huntington’s disease – mental deterioration and uncontrollable movements

17. Codominant Alleles Disease
Disorders that are caused when two alleles contribute to the phenotype.
Example
Sickle Cell Disease – red blood cells twisted and bent.
In sickle cell disease, a small change in the DNA of a single gene affects the structure of the protein hemoglobin, causing a serious genetic disorder.

18. Chromosomal Disorders
The most common error in meiosis occurs when homologous chromosomes fail to separate.
This is known as nondisjunction, which means, “not coming apart.”
Example
Down Syndrome – 3 copies of chromosome 21. (Trisomy 21)
Sex Chromosomal disorder – improper number of either X or Y chromosomes.
Turner’s syndrome – only one X chromosome.
Klinefelter’s – an extra X chromosomes XXY.

19. Chromosomal Disorders
Nondisjunction
Homologous chromosomes fail to separate.
Meiosis I:
Nondisjunction
Nondisjunction causes gametes to have abnormal numbers of chromosomes. The result of nondisjunction may be a chromosome disorder such as Down syndrome.
Meiosis II
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20. Human Molecular Genetics
DNA Analysis – biologist study DNA to better understand ourselves.
Allele testing – testing to see if an individual carries an allele that might carry a genetic disorder.
DNA fingerprinting – DNA is unique to each person, so it can be used to identify individuals.
Human Genome – the mapping of all approx. 6 billion base pairs, that make up human DNA.