Download 7.4 Human Genetics and Pedigrees * Pedigree

7.4 Human Genetics and Pedigrees
KEY CONCEPT
A combination of methods is used to study human
genetics.
7.4 Human Genetics and Pedigrees
Human genetics follows the
patterns seen in other organisms.
- phenotype may be influenced
by the environment (such as
nutrition)
7.4 Human Genetics and Pedigrees
* Types of traits
1. single-gene traits = have 1 allele (dominant
or recessive) that determines phenotype
Ex: widow’s peak (dominant trait), straight
hairline (recessive trait)
7.4 Human Genetics and Pedigrees
2. polygenic traits = phenotype based on
multiple alleles
Ex: eye color
7.4 Human Genetics and Pedigrees
* Sex Linked Disorders
- only females can be carriers
- since females have more than one X chromosome,
the normal allele masks the disorder that is located on
the other X chromosome
- males do not have another X, so they cannot be
carriers – if their X carries the disorder, they will have
the disorder
Ex: hemophilia (was most noted in the family of
Queen Victoria)
7.4 Human Genetics and Pedigrees
- hemophilia is a rare
bleeding disorder
Y
X
- people affected by
hemophilia have a
deficient protein/factor
needed for blood to clot
7.4 Human Genetics and Pedigrees
* Pedigree
- is a chart that is used to trace the phenotypes and
genotypes within a family
(this is different from a Punnett square, which is
used to determine the probable outcome from
a cross)
7.4 Human Genetics and Pedigrees
- male =
marriage
- female =
- carrier =
- shows phenotype =
- when enough family phenotypes are
known, genotypes can be inferred
7.4 Human Genetics and Pedigrees
* Pedigrees can be used to trace autosomal and sex-linked
disorders
- looking at a pedigree - if there are about the same
number of males and females who have the
phenotype, then the gene is most likely on an autosome
- if the phenotype is more common in males, then the
gene is most likely on a sex chromosome (on the X
chromosome – remember, they don’t have a back up!)
7.4 Human Genetics and Pedigrees
- genotype can be inferred based on the pattern of
inheritance (seeFig. 7.14, p.215)
Fig 7.14
-Ex: work your way
up…start with the children
- if only 2 of the 3 children
express the widow’s peak
phenotype (#8 and #10) =
autosomal dominant;
therefore one parent must
be heterozygous (#4) and
the other must be
homozygous recessive (#3)
7.4 Human Genetics and Pedigrees
- sex-linked genes can also be traced
Ex: colorblindness
female carrier x male not colorblind
XM
Y
XM
XM XM
X MY
Xm
X M Xm
Xm Y
carrier daughter
X M = dominant
Xm = recessive
colorblindness is a recessive sexlinked disorder
these 2 kids are not affected
colorblind son
7.4 Human Genetics and Pedigrees
* Karyotype
- is used to study human genetics
XY
7.4 Human Genetics and Pedigrees
~ is a picture of all the chromosomes in a cell
~ are stained so they can be compared and studied (each
chromosome stains differently); are used by scientists to
identify:
7.4 Human Genetics and Pedigrees
1. - extra chromosomes
2. - missing chromosomes
3. - chromosome abnormalities
4. - locate a gene
5. - estimate the distances between genes on a
chromosome
7.4 Human Genetics and Pedigrees
- Down Syndrome (also called Trisomy 21)
~ caused by having an extra chromosome #21
Chris Burke of Life Goes On –
he is the first actor with Down’s
Syndrome to star on TV
7.4 Human Genetics and Pedigrees
Huntington’s Disease
- autosomal dominant
disorder
- caused by a genetic defect
on chromosome 4
- symptoms are displayed at
around age 30 – 40
- causes nerve cells in parts of
the brain break down