Transcript Sex-linked genes - Red Hook Central Schools

Chapter 14.1
Human Chromosomes
Karyotypes
• A karyotype is a picture of the complete diploid
set of chromosomes grouped in homologous
pairs and arranged in decreasing size order.
• Genome – the full set of genetic information that
an organism carries
– the human genome consists of 20,000 – 25,000
genes
– The human genome was studied, as was the genome
of E. coli, the fruit fly, and laboratory mouse
– This project began in 1990, was expected to last 15
years, was completed in 2003.
Sex Chromosomes & Autosomal
Chromsomes
• Two of the 46 chromosomes in the human
genome are sex chromosomes.
• These determine gender
• Females have XX – the X chromosome consists
of more than 1200 genes
• Males have XY – the Y chromosome consists of
about 140 genes
• Autosomes (autosomal chromosomes) – 44
chromosomes not associated with sex
chromosomes
Transmission of Human Traits
• Dominant and Recessive Alleles
• Codominance and Multiple Alleles
• Sex-linked Inheritance – genes located on the X
and Y chromosomes show a pattern of
inheritance called sex-linkage
• Sex-linked genes – located on sex chromsomes
– Genes located on the Y are only found in males
– Recessive sex-linked genes are more common in
males than females. Why?
• Males have only 1 X. For females with 2 X’s, they need 2
recessive alleles to express the gene.
X-Chromosome Inactivation
• Since females have 2 X chromosomes, most of
the genes in one of the X’s are switched off in
the cells.
• This forms what’s called the Barr body in
somatic cells – a dense region in the nucleus
which is the inactive X chromosome
• This random inactivation in the cells accounts for
coat color in cats – males will have only one
color – females can have many (calico)
Pedigrees
• A chart that shows
the presence or
absence of a trait
according to the
relationships between
parents, siblings, and
offspring (and other
family members).
Ch. 14.2
Human Genetic Disorders
Disorders caused by individual
genes
• Sickle cell disease – defective allele for one of the
proteins in hemoglobin – the oxygen-carrying compound
in red blood cells
– Cells have a sickle shape, clump together, are less flexible, and
are less soluble
– Sickle cell disease is a type of incomplete dominance in humans
• Cystic fibrosis – deletion of 3 bases which code for the
amino acid phenylalanine – improper folding of the
protein which gets destroyed
– Recessive allele codes for CF
– cells are unable to transport Cl- ions
– digestive and breathing problems in individuals with the disease
Advantages of genetic disorders
• Sickle-cell disease – individuals with the
allele are resistant to malaria – a
mosquito-borne infection
• Cystic fibrosis – a heterozygous individual
will not have the disease but will be
resistant to typhoid fever – caused by a
bacterium that enters through cells of the
digestive system
Chromosomal disorders
• Nondisjunction – when homologous
chromosomes fail to separate during
anaphase I of meiosis
– Down Syndrome – trisomy 21
• Nondisjunction of the X chromosome:
– Turner’s syndrome – females – only one X
chromosome
– Klinefelter’s syndrome – males – 2 X
chromosomes and a Y chromosome
Ch. 14.3
The Human Genome
Techniques used to study human
DNA
• Cutting DNA: Restriction enzymes cut
DNA into precise pieces (restriction
fragments)
– Each restriction enzyme cuts DNA at a
different sequence of DNA nucleotides
Gel electrophoresis
• Separating DNA: gel electrophoresis – using an
electric current, DNA moves through a gel.
– The smaller the DNA fragment, the farther it moves
– The pattern of bands is based on the fragment size
– Protein-coding regions of DNA are almost identical
among individuals, but long stretches of noncoding
regions of DNA are unique to each individual.
– When these noncoding regions are separated by gel
electrophoresis, a DNA fingerprint is produced.
DNA Fingerprinting
Human Genome Project and the
rise of new fields of study
• Bioinformatics – a new field of studycreating and maintaining databases of
biological information
• Genomics – the study of genomes- genes
and their functions
• Study of the evolution of genes – grouping
protein sequences into families of related
sequences then comparing similar
proteins from different organisms
Quick Lab p. 395
How is Colorblindness Transmitted?
Trial #
1
2
3
4
5
6
7
8
9
10
Colors
Sex of Individual
Number of XLinked Alleles
Class results
Colorblind
boy
Colorblind
girl
Normal
vision boy
Normal
vision girl
38
23
32
20
33
----71
25
----48
45
----77
15
----35