Transcript X-Linked Recessive Inheritance

Genes, Chromosomes, and
Human Genetics
Chapter 13
Why It Matters
 Progeria
13.1 Genetic Linkage and Recombination
 The principles of linkage and recombination
were determined with Drosophila
 Recombination frequency can be used to map
chromosomes
 Widely separated linked genes assort
independently
Chromosomes
 Genes
• Sequences of nucleotides in DNA
• Arranged linearly in chromosomes
Linked Genes
 Genes carried on the same chromosome
• Linked during transmission from parent to
offspring
• Inherited like single genes
• Recombination can break linkage
Drosophila melanogaster
 Fruit fly
• Model organism for animal genetics
• Compared to Mendel’s peas
• Used to test linkage and recombination
Gene Symbolism
 Normal alleles (wild-type)
• Usually most common allele
• Designated by “+” symbol
• Usually dominant
Wild-type
+ = red eyes
+ = normal wings
Mutant
pr = purple
vg = vestigial wings
Genetic Recombination
 Alleles linked on same chromosome exchange
segments between homologous chromosomes
 Exchanges occur while homologous
chromosomes pair during prophase I of meiosis
Recombination Frequency
 Amount of recombination between two genes
reflects the distance between them
 The greater the distance, the greater the
recombination frequency
• Greater chance of crossover between genes
Chromosome Maps
 Recombination frequencies used to determine
relative locations on a chromosome
 Linkage map for genes a, b, and c:
 1 map unit = 1% recombination = 1 centimorgan
Recombination Occurs Often
 Widely separated
linked genes often
recombine
• Seem to assort
independently
• Detected by testing
linkage to genes
between them
13.2 Sex-Linked Genes
 In both humans and fruit flies, females are XX,
males are XY
 Human sex determination depends on the Y
chromosome
13.2 (cont.)
 Sex-linked genes were first discovered in
Drosophila
 Sex-linked genes in humans are inherited as
they are in Drosophila
 Inactivation of one X chromosome evens out
gene effects in mammalian females
Sex Chromosomes
 Sex chromosomes determine gender
• X and Y chromosomes in many species
• XX: female
• XY: male
 Other chromosomes are called autosomes
Human Sex Chromosomes
 Human X chromosome
• Large (2,350 genes)
• Many X-linked genes are nonsexual traits
 Human Y chromosome
• Small (few genes)
• Very few match genes on X chromosome
• Contains SRY gene
• Regulates expression of genes that trigger male
development
Sex Linkage
 Female (XX): 2 copies of X-linked alleles
 Male (XY): 1 copy of X-linked alleles
 Only males have Y-linked alleles
Sex Linkage
 Males have only one X chromosome
• One copy of a recessive allele results in
expression of the trait
 Females have two X chromosomes
• Heterozygote: recessive allele hidden (carrier)
• Homozygote recessive: trait expressed
Eye Color Phenotypes in Drosophila
 Normal wild-type: red eye color
 Mutant: white eye color
Human Sex-Linked Genes
 Pedigree chart show genotypes and phenotypes
in a family’s past generations
 X-linked recessive traits more common in males
• Red-green color blindness
• Hemophilia: defective blood clotting protein
Inheritance of Hemophilia
 In descendents of Queen Victoria of England
X Inactivation (1)
 Dosage compensation
• In female mammals, inactivation of one X
chromosome makes the dosage of X-linked
genes the same as males
 Occurs during embryonic development
X Inactivation (2)
 Random inactivation of either X chromosome
 Same X chromosome inactivated in all
descendents of a cell
 Results in patches of cells with different active X
chromosomes
Calico Cats
 Heterozygote female (no male calico cats)
Barr Body
 Tightly coiled condensed X chromosome
 Attached to side of nucleus
 Copied during mitosis but always remains
inactive
13.3 Chromosomal Alterations That
Affect Inheritance
 Most common chromosomal alterations:
deletions, duplications, translocations, and
inversions
 Number of entire chromosomes may also
change
Chromosomal Alterations (1)
 Deletion: broken
segment lost from
chromosome
 Duplication:
broken segment
inserted into
homologous
chromosome
Chromosomal Alterations (2)
 Translocation:
broken segment
attached to
nonhomologous
chromosome
 Inversion: broken
segment reattached
in reversed
orientation
Nondisjunction (1)
 Failure of homologous pair separation during
Meiosis I
Nondisjunction (2)
 Failure of chromatid separation during
Meiosis II
Changes in Chromosome Number
 Euploids
• Normal number of chromosomes
 Aneuploids
• Extra or missing chromosomes
 Polyploids
• Extra sets of chromosomes (triploids, tetraploids)
• Spindle fails during mitosis
Aneuploids
 Abnormalities usually prevent embryo
development
 Exception in humans is Down syndrome
• Three copies of chromosome 21 (trisomy 21)
• Physical and learning difficulties
• Frequency of nondisjunction increases as women
age
Polyploids
 Common in plants
• Polyploids often hardier and more successful
• Source of variability in plant evolution
 Uncommon in animals
• Usually has lethal effects during embryonic
development
13.4 Human Genetics and Genetic
Counseling
 In autosomal recessive inheritance,
heterozygotes are carriers and homozygous
recessives are affected by the trait
 In autosomal dominant inheritance, only
homozygous recessives are unaffected
13.4 (cont.)
 Males are more likely to be affected by X-linked
recessive traits
 Human genetic disorders can be predicted, and
many can be treated
Modes of Inheritance
 Autosomal recessive inheritance
 Autosomal dominant inheritance
 X-linked recessive inheritance
Autosomal Recessive Inheritance
 Males or females carry a recessive allele on an
autosome
 Heterozygote
• Carrier
• No symptoms
 Homozygote recessive
• Shows symptoms of trait
Autosomal Dominant Inheritance
 Dominant gene is carried on an autosome
 Homozygote dominant or heterozygote
• Show symptoms of the trait
 Homozygote recessive
• Normal
X-Linked Recessive Inheritance
 Recessive allele carried on X chromosome
 Males
• Recessive allele on X chromosome
• Show symptoms
 Females
• Heterozygous carriers, no symptoms
• Homozygous, show symptoms
Genetic Counseling
 Identification of parental genotypes
• Construction of family pedigrees
• Prenatal diagnosis
 Allows prospective parents to reach an informed
decision about having a child or continuing a
pregnancy
Genetic Counseling Techniques
 Prenatal diagnosis tests cells for mutant alleles
or chromosomal alterations
 Cells obtained from:
• Embryo
• Amniotic fluid around embryo (amniocentesis)
• Placenta (chorionic villus sampling)
 Postnatal genetic screening
• Biochemical and molecular tests
13.5 Nontraditional Patterns of
Inheritance
 Cytoplasmic inheritance follows the pattern of
inheritance of mitochondria or chloroplasts
 In genomic imprinting, the allele inherited from
one of the parents is expressed while the other
allele is silent
Cytoplasmic Inheritance
 Genes carried on DNA in mitochondria or
chloroplasts
 Cytoplasmic inheritance follows the maternal
line
• Zygote’s cytoplasm originates from egg cell
Cytoplasmic Inheritance
 Mutant alleles in organelle DNA
• Mendelian inheritance not followed (no
segregation by meiosis)
• Uniparental inheritance from female
Cytoplasmic Inheritance
 Inheritance of variegation in Mirabalis
Genomic Imprinting
 Expression of an allele is determined by the
parent that contributed it
• Only one allele (from either father or mother) is
expressed
 Other allele is turned off (silenced)
• Often, result of methylation of region adjacent to
gene responsible for trait