Molec and Cell 4 Flashcards
Homunculus
sperm contained fully formed “little
people
Like begets like: offspring are like their parents
Everything from the egg: females controlled all traits
Paternal heredity
males control all traits
Blending inheritance
offspring were the average
between both parents
Inheritance of acquired characters
changes happening to parents could be passed on to offspring
Pangenesis
the idea that particles called “gemmules”
carry the traits we inherit.
Gregor Johann Mendel
Considered the father of
genetics
Mendel discovered the basic principles of heredity by breeding garden peas in carefully planned experiments
character
A heritable feature that varies among individuals (such as flower color)
trait
Each variant for a character, such as purple or white color for flowers
truebreeding
plants that produce offspring of the same variety when they self-pollinate
hybridization
a process of mating two contrasting, true-breeding varieties
P generation
true-breeding parents
F1 generation
The hybrid offspring of the P generation
F2 generation
When F1 individuals self-pollinate or cross-pollinate with other F1 hybrids
Phenotype
the characteristic version of a trait we actually see. Example: purple flower or blue eyes
Alleles
specific versions of a “hereditary particle” (today= versions of a gene)
Laws of Probability first concept
alternative versions of genes account for variations in inherited characters
Laws of Probability second concept
that for each character an organism inherits two alleles, one from each parent
Probability basics multiplication rule
For independent events A and B, the probability (P) of both occurring (A and B) is (PA x PB)
Probability basics sum rule
For mutually exclusive events A and B, the probability (P) that at least one occurs (A or B) is (PA + PB)
Laws of Probability third concept
if the two alleles at a locus differ, then one (the dominant allele) determines the organism’s appearance, and the other (the recessive allele) has no noticeable effect on appearance
Laws of Probability four concept
known as the Law of Segregation, states that the two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes
monohybrids
individuals that are heterozygous for one character
monohybrid cross
A cross between such heterozygotes
dihybrids
Crossing two true-breeding parents differing in two characters, heterozygous for both characters
Mendelian Ratios 3:1
3 Dominant Phenotype
1 Recessive Phenotype
Mendelian Ratios 9:3:3:1
9 Double Dominant Phenotypes
3 Dominant/Recessive Phenotypes
3 Recessive/Dominant Phenotypes
1 Double Recessive Phenotypes
Complete dominance
phenotypes of the heterozygote and dominant homozygote are identical
incomplete dominance
the phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties (snapdragon colors)
codominance
two dominant alleles affect the phenotype in separate, distinguishable ways (blood type)
The alleles for ABO blood groups
I^A, I^B, i
Phenotype A blood type
I^A I^A, I^A i
Phenotype B blood type
I^B I^B, I^B i
Phenotype O blood type
ii
Phenotype AB blood type
I^A I^B
Homeotic mutants
affect development and potentially many characters
Sex Linked Traits
Traits located on X, Y chromosomes or sex chromosomes
Human Sex Linked Traits two types
Recessive lethal
Dominant lethal
Carriers
heterozygous individuals who carry the recessive allele but are phenotypically normal
Risks of incest
If a recessive allele that causes a disease is rare, it is unlikely that two carriers will meet and mate
Consanguineous matings (that is, between close relatives) increase the chance that both parents of a child carry the same rare allele
Cystic fibrosis
most common lethal genetic disease in the United States, striking one out of every 2,500 people of European descent
allele results in defective or absent chloride transport channels in plasma membranes, leading to a buildup of chloride ions outside the cell
Symptoms include mucus buildup in some internal organs and abnormal absorption of nutrients in the small intestine
Sickle-cell disease
affects one out of 400 African-Americans
caused by the substitution of a single amino acid in the hemoglobin protein in red blood cells
In homozygous individuals, all hemoglobin is abnormal (sickle-cell)
Symptoms include physical weakness, pain, organ damage, and even paralysis
Heterozygotes for sickle cell trait
usually healthy but may suffer some symptoms
About one out of ten African-Americans has sickle-cell trait, an unusually high frequency
Heterozygotes are less susceptible to the malaria parasite, so there is an advantage to being heterozygous in regions where malaria is common
Dominantly Inherited Disorders
causes a lethal disease are rare and arise by mutation
Huntington’s disease
a degenerative disease of the nervous system
The disease has no obvious phenotypic effects until the individual is about 35 to 40 years of age
Once the deterioration of the nervous system begins, the condition is irreversible and fatal
Pairs of Unit Factors, or Genes
The first concept is that alternative versions of genes account for variations in inherited characters
Law of Dominance
The third concept is that if the two alleles at a locus differ, then one (the dominant allele) determines the organism’s appearance, and the other (the recessive allele) has no noticeable effect on appearance
Equal Segregation of Alleles
the two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes
Independent Assortment
How one Allele segregates has no affect on how another Allele segregates
Linked Genes Violate the Law of Independent Assortment
Epistasis
One gene controls the expression of another
Dog color, One gene determines the pigment color (with alleles B for black and b for brown)
The other gene (with alleles E for color and e for no color) determines whether the pigment will be deposited in the hair
modified versions of 9:3:3:1
Polygenic Inheritance
Quantitative characters are those that vary in the population along a continuum
Quantitative variation usually indicates polygenic inheritance
polygenic inheritance
an additive effect of two or more genes on a single phenotype (skin color)
chromosome theory of inheritance
Mendelian genes have specific loci (positions) on chromosomes
Chromosomes undergo segregation and independent assortment
first solid evidence associating a specific gene with a specific chromosome
Thomas Hunt Morgan, an embryologist
Morgan’s experiments with fruit flies provided convincing evidence that chromosomes are the location of Mendel’s heritable factors
wild type
normal, phenotypes that were common in the fly populations
mutant phenotypes
Traits alternative to the wild type
chromosomal basis of sex determination
two varieties of sex chromosomes: a larger X chromosome and a smaller Y chromosome
Only the ends of the Y chromosome have regions that are homologous with the X chromosome
SRY gene on the Y chromosome
Codes for the development of testes through increase of testosterone
sex-linked gene
A gene located on either sex chromosome
In humans, sex-linked usually refers to a gene on the larger X chromosome
For a recessive sex-linked trait to be expressed
A female needs two copies of the allele
A male needs only one copy of the allele
Sex-linked recessive disorders are much more common in males than in females
Some disorders caused by recessive alleles on the X chromosome in humans
Color blindness
Duchenne muscular dystrophy
Hemophilia
inactive X in mammalian females
one of the two X chromosomes in each cell is randomly inactivated during embryonic development and condenses into a Barr body
How many genes are on a chromosome
hundreds or thousands of genes
linked genes
Genes located on the same chromosome that tend to be inherited together
genetic recombination
the production of offspring with combinations of traits differing from either parent
parental types
Offspring with a phenotype matching one of the parental phenotypes
recombinant types, or recombinants
Offspring with nonparental phenotypes (new combinations of traits)
Chiasmata
crossing over of homologous chromosomes
some process must sometimes break the physical connection between genes on the same chromosome
genetic map
an ordered list of the genetic loci along a particular chromosome
A crossover event and loci of gene on chromosome relation
the farther apart two genes are, the higher the probability that a crossover will occur between them and therefore the higher the recombination frequency
linkage map
a genetic map of a chromosome based on recombination frequencies
one map unit, or centimorgan
Distances between genes can be expressed as map units, represents a 1% recombination frequency
Map units indicate relative distance and order, not precise locations of genes
Genes that are far apart on the same chromosome can have a recombination frequency near 50%
Cytogenetic maps
the positions of genes with respect to chromosomal features
Large-scale chromosomal alterations
often lead to spontaneous abortions (miscarriages) or cause a variety of developmental disorders
nondisjunction
pairs of homologous chromosomes do not separate normally during meiosis
one gamete receives two of the same type of chromosome, and another gamete receives no copy
Aneuploidy
the fertilization of gametes in which nondisjunction occurred
Offspring with this condition have an abnormal number of a particular chromosome
Two types of Aneuploidy
monosomic zygote
trisomic zygote
monosomic
zygote has only one copy of a particular chromosome
trisomic
three copies of a particular chromosome
Polyploidy
a condition in which an organism has more than two complete sets of chromosomes
Triploidy
(3n) is three sets of chromosomes
Tetraploidy
(4n) is four sets of chromosomes
Polyploidy is common in plants, but not animals
Polyploids are more normal in appearance than aneuploids
four types of changes in chromosome structure
Deletion
Duplication
Inversion
Translocation
Deletion
removes a chromosomal segment
Duplication
repeats a segment
Inversion
reverses a segment within a chromosome
Translocation
moves a segment from one chromosome to another
Down syndrome
an aneuploid condition that results from three copies of chromosome 21
It affects about one out of every 700 children born in the United States
Klinefelter syndrome
the result of an extra chromosome in a male, producing XXY individuals
Monosomy X
Turner syndrome
produces X0 females, who are sterile; it is the only known viable monosomy in humans
syndrome cri du chat (“cry of the cat”)
results from a specific deletion in chromosome 5
A child born with this syndrome is mentally retarded and has a catlike cry; individuals usually die in infancy or early childhood
Certain cancers, including chronic myelogenous leukemia (CML)
caused by translocations of chromosomes
genomic imprinting
involves the silencing of certain genes that are “stamped” with an imprint during gamete production
the result of the methylation (addition of –CH3) of DNA
