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14–2 Human Chromosomes
14-2 Human Chromosomes
Photo credit: Richard Hutchings/Photo Researchers, Inc.
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Human Chromosomes
Human Karyotype -
How do we read a karyotype?
How many chromosomes?
How many pairs of homologous chromosomes?
Which chromosomes determines the sex of an individual?
These human chromosomes have been cut out of a photograph and arranged to form a karyotype. Photo credit: ©CNRI/Science Photo Library/Photo Researchers, Inc.
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Human Chromosomes
The 23rd pair of chromosomes are know as the 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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Sex-Linked Genes
What is the probability of producing an offspring of a certain sex?
Do Punnett square 50/50 chance
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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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Sex-Linked Genes
Genes located on the sex chromosomes are called sex-linked genes, some of them are disorders like colorblindness or muscular dystrophy. Most sex-linked disorders are recessive.
**Why are sex-linked disorders more common in males than in females?**
For a recessive allele to be expressed in females, there must be two copies of the allele, one on each of the two X chromosomes.
Males have just one X chromosome. Thus, all X-linked alleles are expressed in males, even if they are recessive.
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Sex-Linked Genes
Colorblindness
The gene associated with color vision are located on the X chromosome.
In males, a defective version of any one of these genes produces colorblindness.
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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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Sex-Linked Genes
Hemophilia
The X chromosome also carries genes that help control blood clotting. A recessive allele in either of these two genes may produce hemophilia.
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.
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10. Chromosomal Disorders
What happens if an error occurs during Meiosis?
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11. 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.”, resulting in either more or less chromosomes.
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12. 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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13. Chromosomal Disorders
Down Syndrome
If two copies of an autosomal chromosome fail to separate during meiosis, an individual may be born with three copies of a chromosome.
Down syndrome involves three copies of chromosome 21.
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14. Chromosomal Disorders
Down syndrome produces mild to severe intellectual disabilities. It is characterized by:
increased susceptibility to many diseases
higher frequency of some birth defects
Down Syndrome Karyotype
This karyotype is from a person with Down syndrome. Down syndrome causes mental retardation and various physical problems. People with Down syndrome can, however, lead active, happy lives. Photo credit: ©Dr. Dennis Kunkel/CNRI/Phototake
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15. Chromosomal Disorders
Sex Chromosome Disorders – Abnormal Number of sex chromosomes.
In females, nondisjunction can lead to Turner’s syndrome.
A female with Turner’s syndrome usually inherits only one X chromosome (karyotype 45,X).
Women with Turner’s syndrome are sterile.
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14–2
Which of the following genotypes indicates an individual who is a carrier for colorblindness?
XCX
XCXc
XcY
XCY
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14–2
Colorblindness is much more common in males than in females because
the recessive gene on the male’s single X chromosome is expressed.
genes on the Y chromosome make genes on the X chromosome more active.
females cannot be colorblind.
colorblindness is dominant in males and recessive in females.
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14–1 Human Heredity
14-1 Human Heredity
Photo credit: Richard Hutchings/Photo Researchers, Inc.
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Human Traits
Pedigree Charts 
A pedigree chart shows the relationships within a family.
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Human Traits
A square 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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Pedigree Chart
1. Which members of the family above are afflicted with Huntington’s Disease? _________________________________
2. There are no carriers for Huntington’s Disease- you either have it or you don’t.
With this in mind, is Huntington’s disease caused by a dominant or recessive trait? ____________________________
3. How many children did individuals I-1 and I-2 have? _______________________________________________
4. How many girls did II-1 and II-2 have? ______________ How many have Huntington’s Disease? ________________
5. How are individuals III-2 and II-4 related? ________________________ I-2 and III-5? _________________________
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Human Genes
Recessive Alleles
The presence of a normal, functioning gene is revealed only when an abnormal or nonfunctioning allele affects the phenotype.
Many disorders are caused by autosomal recessive alleles. (two lowercase letters – aa)
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Human Genes
This table shows the major symptoms of some well-known genetic disorders.
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Human Genes
Dominant Alleles
The effects of a dominant allele are expressed even when the recessive allele is present. (Heterozygous = Aa), the recessive allele is hidden, the individual is a carrier.
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Human Genes
This table shows the major symptoms of some well-known genetic disorders.
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From Gene to Molecule
From Gene to Molecule
How do small changes in DNA cause genetic disorders?
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From Gene to Molecule
In both cystic fibrosis and sickle cell disease, a small change in the DNA of a single gene affects the structure of a protein, causing a serious genetic disorder.
Cystic Fibrosis
Cystic fibrosis is caused by a recessive allele.
Sufferers of cystic fibrosis produce a thick, heavy mucus that clogs their lungs and breathing passageways.
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How is a genetic disorder different than a infectious disease? How are they the same? Create a Venn Diagram
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14–1
A chromosome that is not a sex chromosome is know as a(an)
autosome.
karyotype.
pedigree.
chromatid.
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14–1
Whether a human will be a male or a female is determined by which
sex chromosome is in the egg cell.
autosomes are in the egg cell.
sex chromosome is in the sperm cell.
autosomes are in the sperm cell.
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