LESSON 2 Flashcards
branch of biology that deals with heredity and variation of organisms.
Genetics
Carry the genetic information or hereditary information (genes)
○ Arrangement of nucleotides in DNA
■ DNA -> RNA -> Protein (Central
Dogma)
Chromosomes
contain DNA that codes for the same genes.
Homologous chromosomes
T or F: in Homologous chromosome, both chromosomes have all the same genes in the same locations (represented with colored strips), but different “versions” of those genes (represented by different shades of each color).
True
a unit of heredity; a section of DNA sequence encoding a single protein
Gene
the entire set of genes in an organism
Genome
two genes that occupy the same position on homologous chromosomes and that cover the same trait (like flavors a trait)
Alleles
a fixed location on a strand of DNA where a gene or one of its alleles is located.
Locus
the genetic makeup of an organisms
Genotype
the physical appearance of an organism (Genotype + environment)
Phenotype
having identical genes (one from each parent) for a particular characteristic. ○ Example: Purple flower (PP) or White flower (ww)
Homolozygous
having two different genes for a particular characteristic.
○ Example: Purple flower (Pp) or White flower (Ww)
Heterozygous
a trait in which a gene is carried on a sex chromosome
Sex-linked trait
traits controlled by genes on one of 22 pairs of autosomes
Autosomal trait
the allele of a gene that masks or suppresses the expression of an alternate allele; the trait appears in the heterozygous condition.
○ When a dominant allele is present, the dominant will be manifested and masked the recessive allele
Dominant allele
an allele that is masked by a dominant allele; does not appear in the heterozygous condition, only in homozygous.
Recessive allele
a genetic cross involving a single pair of genes (one trait); parents differ by a single trait.
Monohybrid cross
a genetic cross between two different genes that differ in two observed traits
Dihybrid cross
P
Parental generation
F1
First filial generation; offspring from a genetic cross.
F2
Second filial generation of a genetic cross
○ Usually, a particular trait is manifested here in second filial generation after a genetic cross of homozygous alleles, which was not shown in the first filial generation
Mendelian gene/trait
Single gene disease
is the most common expression of a particular allele combination in a population. The wild type allele may be recessive or dominant.
Wind type phenotype
is a variant of a gene’s expression that arises when the gene undergoes a change, or mutation.
Mutant phenotype
Known as an illness that typically begins in early adulthood, causing uncontrollable movements and changes in behavior and thinking (cognition), with death 15 to 20 years later.
Huntington disease
Is a dominant, which means that each child of an affected individual need inherit only one copy of the mutant gene to develop the disease.
● However, 10 percent of people who have HD are under age 20. They have juvenile ________
Huntington disease (caused by an HD Gene which produces huntington protein)
T or F: Even if only one allele is passed, if it is a dominant allele, the disease can be carried through generations
True
Traits of two parents “blend” together and inherited by the offspring
Blending theory of inheritance
According to _______ , traits do not blend together but rather traits are inherited separately
Mendel
One of his experiments involved crossing a purple flowers with white flowers (Monohybrid Cross)
Mendel’s experiment
● Also known as the Gene Idea
● Inheritance involves the passing of discrete units of inheritance, or genes, from parent to offspring.
Particulate theory of inheritance by Mendel
● Proposed by Walter Sutton and proved by Thomas Morgan using fruit fly
● Genes are present within chromosomes inside the cell
● Genes and chromosomes are in pairs in diploid cells
Chromosome theory of inheritance
Patterns of inheritance
Principle of dominance, segregation and independent assortment
● One allele masks another, one allele was dominant over the other in the F1 generation.
● Whenever a dominant allele (even if only one), is present, the dominant trait will be manifested and masked the expression of recessive allele
Principle of dominance
When gametes are formed, the pairs of hereditary factors (genes) become separated, so that each sex cell (egg/sperm) receives only one kind of gene.
Principle of segregation
● Genes located on different chromosomes will be inherited independently of each other.
● Both genes from your father and mother are not inherited at the same time
● Example:
○ There are 7 traits in total from your mother and
father
○ Not all of the traits will be inherited by your child
all together at once
○ There will be combinations of different traits
from your mother and father (height from mother and eye color from father)
Principle of independent assortment
Mendel noted that short plants crossed to other short plants were “true-breeding,” always producing the same phenotype, in this case short plants.
● The crosses of tall plants to each other were more confusing. Some tall plants were true-breeding, but others crossed with each other yielded short plants in about one-quarter of the next generation. In some plants, tallness appeared to mask shortness.
● One trait that masks another is dominant; the masked trait is recessive.
On first principle law
Some dominantly inherited diseases are said to be due to a “gain-of-function”, because they result from the action of an abnormal protein that interferes with the function of the normal protein.
● Can be best described by the Huntington Disease
On the Dominance Principle
results from a gain-of-function in which the dominant mutant alleles encodes an abnormally long protein that prevents the normal protein from functioning in certain brain cells.
Huntington disease
A single gene on chromosome 15,______, confers eye color by controlling melanin synthesis.
OCA2
But inheritance of eye color is more complicated than this. Near the OCA2 gene on chromosome 15 is a second gene, ________, that controls expression of the OCA2 gene. A recessive allele of _______ abolishes control over OCA2, the blue eyes result. A person must inherit two copies of the recessive allele in ________ to have blue eyes.
HERC2
● This is the case for sickle cell disease.
● Carriers can develop a life-threatening breakdown of
muscle if exposed to the combination of environmental heat, intense physical activity, and dehydration. Several college athletes died from these symptoms, prompting sport authorities to begin testing athletes for sickle cell disease carrier status.
● This is called sickle cell trait.
ON HETEROZYGOSITY IN A DOMINANT TRAIT
You have both the sickle-shaped RBC
and normal-shaped RBC
Heterozygous
An individual with two different recessive alleles for the same gene is termed a
Compound heterozygous
Child has a recessive allele from each parent with the variant located at a different position within the same gene
Compound heterozygotr
As molecular genomic techniques became available in the _______ & ________ , it became possible to explain a range of disorders in heterozygotes carrying one copy of one of the classic mutations for phenylketonuria.
1980s and 1990s
● Because ___________ was the first genetic disorder for which mass post-natal genetic screening was available, beginning in the early 1960s, atypical cases were detected almost immediately.
● Molecular analysis of the genome was not yet possible, but protein sequencing revealed cases caused by compound heterozygosity.
phenylketonuria
A metabolic disorder involving deficiency of enzymes that metabolize phenylalanine
○ If not metabolized, it may cause mental retardation
● It should be detected at young age
● Part of newborn screening test
Phenylketonuria
In addition to its classic infantile form, ________ disease may present in juvenile or adult-onset forms, often as the result of compound heterozygosity between two alleles, one that causes the classic infantile disease in homozygotes and another that allows some residual HEXA enzyme activity.
Tay Sachs
an individual with one allele for hemoglobin S and one allele for hemoglobin C would still develop the disease, despite being heterozygous for both genes.
sickle cell anemia
result from inheritance of the ____________ gene in a compound heterozygous manner with other mutant beta globin genes.
sickle cell syndrome
said to arise from a “loss-of-function” because it usually prevents the production or activity of the normal protein
ON THE RECESSIVENESS PRINCIPLE
The basis of an inborn error of metabolism is easy to picture. These diseases are typically recessive because the half-normal amount of the enzyme that a carrier produces is usually sufficient to maintain health.
tend to be more severe, and produce symptoms earlier.
Most autosomal _______ conditions appear unexpectedly in families, because they are transmitted silently, through heterozygotes (carriers).
Marriage between relatives introduces consanguinity, which means “shared blood”—a figurative description, because genes are not passed in blood. Alleles inherited from shared ancestors are said to be “identical by descent.”
ON THE RECESSIVENESS PRINCIPLE
Males and females affected, with equal frequency
Successive generations affected until no one inherits the mutation
Affected individual has an affected parent, unless he or she has a de novo mutation
De novo mutation
Autosomal dominant
Males and females affected, with equal frequency
Can skip generations
The trait is not manifested unless the alleles are homozygous
Affected individual has parents who are affected or are carriers (heterozygotes)
Autosomal recessive
If inherited mutant alleles from both parents, a _______ allele will manifest
recessive
An individual with two different alleles is __________—what Mendel called “non-true-breeding” or “hybrid.”
heterozygous
Mendel’s first law
Law of segregation
reflects the actions of chromosomes and the genes they carry during meiosis. Because a gene is a long sequence of DNA, it can vary in many ways.
Law of segregation
Mendel’s 2nd law
law of independent assortment
for two genes on different chromosomes, the inheritance of one gene does not influence the chance of inheriting the other gene. The two genes are said to “independently assort” because they are packaged into gametes at random.
Two genes that are far apart on the same chromosome (see discussion on linkage) also appear to independently assort, because so many crossovers take place between them that it is as if they are part of separate chromosomes
law of independent assortment
Principle 1 - Test cross/Pedigree analysis Law 1 (Law of Segregation) - Punett square; Single test cross/monohybrid
Law 2 (Law of Independent Assortment)- Dihybrid Cross
True
A useful tool to do genetic crosses
Looks like a windowpane
Used to determine the probability of outcome of offspring
Punnett square
a lethal genetic disease affecting Caucasians.
● Caused by mutant recessive gene carried by 1 in 20 people of European descent
○ To be manifested = Homozygous recessive allele
Cystic fibrosis
disease affects transport in tissues – mucus is accumulated in lungs, causing infections.
Cystic fibrosis
A fatal genetic disorder that causes the progressive breakdown of nerve cells in the brain
HD
Mating that involve parents that differ in two genes (two independent traits)
Dihybrid cross
When you have an individual with an unknown genotype, you do a
test cross.
Cross with a homozygous recessive individual
Test cross
genes close on a chromosome are packaged into the same gametes and are said to be
linked or linkage
● _______ in genetics refers to the transmission of genes on the same chromosome.
● _______ genes do not assort independently and do not produce Mendelian ratios for crosses tracking two or more genes.
Linkage
○ He observed that the Drosophila’s fly does not
follow Mendelian inheritance
○ He identified more than 50 genes on Drosophila’s 4 pairs of chromosomes.
○ He discovered that many seemed to be “linked” together
Thomas Morgan’s Research
map based on the frequencies of recombination between markers during crossover of homologous chromosome. A map of the genes on a chromosome based on linkage analysis. map does not show the physical distances between genes but rather their relative position, as determined by how after two Gene loci are inherited together.
Linkage map
Linkage maps were first developed by, a student of Thomas Hunt Morgan.
Alfred Sturtevant
LD blocks, called ________, could be used to track genes in populations
haplotypes
a combination of alleles at multiple linked loci that are transmitted together.
Haplotype
The X-linked recessive trait (recessive allele), the blood clotting disease __________ is also known as Christmas disease and factor IX (“FIX”) deficiency.
hemophilia B
● Rare neurodevelopmental disorder
● Progressive, not degenerative
● Typical child with this syndrome cannot speak, use her
hands, walk, eat, or breath easily.
● Longevity well into adulthood
● No FDA approved treatments.
‘Retr syndrome
affects a structure or function of the body that is present in only males or only females. The gene for such a trait may be autosomal (eg. lactation)
Sex-limited trait
due to genes on autosomes, and the expression of these traits are limited so that normally only one sex is affected.
Sex-limited trait
An example of sex-limited trait, is a sudden rise in blood pressure late in pregnancy. It kills 50,000 women worldwide each year. A study of 1.7 million pregnancies in Norway found that if a man’s first wife had preeclampsia, his second wife had double the risk of developing it
Preeclampsia
are autosomal traits that are influenced by sex. If a male has one recessive allele, he will show that trait, but it will take two recessive alleles for the female to show that same trait. One such gene is baldness
Sex-influenced trait
A genotype (allele combination) that causes death is, by strict definition,
Lethal
is lethal by age 3 or 4
Tay sachs
achondroplastic dwarfism
Lethal
Type of relationship between alleles, with a heterozygote phenotype intermediate between the two homozygote phenotypes.
Enzyme deficiencies in which a threshold level is necessary for health illustrate both complete and incomplete dominance.
Incomplete dominance
An example of incomplete dominance that can be observed in carriers on both the molecular and whole-body levels.
FAMILIAL HYPERCHOLESTEROLEMIA (FH)
Different alleles that are both expressed in heterozygote are
Co-dominant
Mendel’s laws can appear not to operate when one gene masks or otherwise affects the phenotype of another.
Epistasis
It results from an interaction between a gene called H and the I gene that confers ABO blood type.
Bombay phenotype under epistasis
A single-gene disease with many symptoms, or a gene that controls several functions or has more than one effect
Pleiotropy
The most common form of this autosomal dominant condition is a defect in an elastic connective tissue protein called fibrillin.
Marfan syndrome
Mutations in different genes that produce the same phenotype lie behind________________. It can occur when genes encode enzymes or other proteins that are part of the same biochemical pathway, or when proteins affect the same body part
genetic heterogeneity.
Abnormal collagen causes fragile bones.
Osteogenesis
● An environmentally caused trait that appears to be inherited is a phenocopy.
● Such a trait can either produce symptoms that resemble those of a known single-gene disease or mimic inheritance patterns by affecting certain relatives.
Phenocopies
● Homeotic genes regulate organ development
● Homeobox genes code for transcription factors
● Hox genes determine the identities of each body segment
● Homeobox genes in humans
True
refers to the inheritance of a trait governed by more than one genes.
● Generally, three or more genes govern the inheritance of polygenic traits. Multiple independent genes have an additive or similar effect on a single quantitative trait.
POLYGENIC TRAITS
