How Did i Get This Face

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How did I get this face? Bio 100 Instructor Deborah Bird Worksheet #5

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Part 3 of worksheet #5

How did I get this face?Mike Tveten

drawings by Chris BondanteModified by Deborah Bird

OBJECTIVES Apply the principles of segregation and independent assortment to the inheritance of human

facial characteristics Determine the phenotypes and genotypes for yourself and your parents Define allele, gene, homozygous, heterozygous Define and give an example of dominant and recessive alleles, codominance, incomplete

dominance, X-linked (sex-linked) traits, and polygenic traits

The purpose of this lab is to illustrate:1. Mendel's Law of Segregation2. Mendel's Law of Independent Assortment3. the relationship between genotype and phenotype4. a variety of patterns of inheritance

INTRODUCTION

As you learned when you studied meiosis in the last unit, you received one half of yourgenes from your mother and the other half from your father. The chromosomes wentthrough segregation from one another when they separated and made sex cells. Eachone of the sex cells carried a gene at a locus. When fertilization took place thehomologous chromosomes were restored and a new set of genetic material was

introduced into the egg from the sperm. The sperm cells were also segregated duringmeiosis and depending upon which egg was fertilized by what sperm actually gives youyour individual genetic compliment called the genotype. Human variation happens notonly because of the large number of traits that exist within our species, but also becauseof the random mixing of alleles that occurs during sexual reproduction. The expressionof the genes which we see physically is called the phenotype.

There are many patterns of inheritance some are relatively simple and straightforward,while others are more complex. Many alleles(forms of a gene) are dominant, whichmeans that they mask or hide the expression of other alleles, called recessivealleles.Recessive alleles are expressed only when their dominant counterparts are not present

in other words, when we inherit two copies of the recessive allele, one from eachparent. Remember, when we have two copies of the same allele, our genotype ishomozygous for that trait.

For example if you have a round face and inherited a R from mom and a R from dad ,you are homozygous dominant for that trait. Dominant traits can be expressed even ifthere is only one. The genotype Rr is still a round face. The large R is dominating thers expression. Even though you are heterozygous (Rr) for the trait. Recessive allelesare expressed only in the homozygous condition (rr).


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If our genotype carries one copy of the dominant trait and one copy of the recessivetrait then it is called heterozygous. In individuals with two different alleles for a traitheterozygous for the trait), neither allele completely masks the presence of the other,and the result is a combination of both. Other traits show continuous variationthroughout a population and do not result in a distinctly dominant or recessivephenotype. These traits are thought to be controlled by many genes and are called

polygenictraits. Another kind of interaction between genes is called epistasis.Anexample of this is the interaction between the gene that codes for red hair and the genethat codes for dark versus blonde. Thus, a person may be a strawberry blonde, or havedark auburn hair, depending on how these two genes interact.

Some traits are carried on the X chromosome, and there is no counterpart on the Ychromosome. Because females have two X chromosomes, a recessive allele might bemasked by the presence of a dominant allele. However, because males have only one Xchromosome, recessive traits are often expressed since there is no other allele to maskthem. These traits are called sex-linked, or more accurately, X-linked. Other traits maybe found only on the Y chromosome, so they are found only in males. These traits, such

as beard growth, are sex-limited , which means their expression is confined to onegender or the other (in this example, males).

Most of the traits used in this activity have been simplified and represent a simplifiedmodel. In this exercise you will be determining your phenotype and guessing at yourgenotype . The last column of the table in worksheet 5 is answered mother, father, ormixture. Before you do this exercise, you need to determine your phenotype, and makeyour best guess about your genotype

Unless you have an identical twin, you are unique! Your appearance (your phenotype) isdetermined by your genes (your genotype). Even people with the same parents haveslightly different genotypes and therefore different phenotypes. (Remember synapsis inmeiosis makes this possible.) Identical twins have identical genotypes because the

fertilized egg splits after fertilization and grows another individual.

Filling out the TableTake a long, hard look at yourself in the mirror or have someone look at you. Then usethe illustrations that follow to determine your phenotype for the illustrated traits, suchas eye color, nose size, and freckles. Record your phenotype for each trait in Table 1 onworksheet number 5.

Then try to determine your genotype, and record your genotype in the table. In a simpledominant/recessive pattern of inheritance, if you show the recessive phenotype, you

know that you are homozygous recessive for that trait. However, if you show thedominant phenotype, you may be homozygous or heterozygous. For this exercise,assume you are heterozygous(that is, you carry both the dominant and recessivealleles, unless the phenotype tells you the genotype. See the example below.

Remember -- PHENOTYPE is the PHYSICAL FEATURES -- what you see, such as"rounded face" or "cleft chin." GENOTYPE is the GENES (alleles) present that producethat phenotype. For example, if the phenotype is "rounded face," then the genotypeis either "RR" or "Rr." You would use Rr for this exercise. For air texture, curly is


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CC, wavy is Cc, straight is cc. In this case we can tell the genotype from thephenotype.

Then, determine what alleles (gene form) will be in your gametes. Remember, if you areheterozygous for a trait such as Rr you can produce gametes with one or the other ofyour alleles, but not both in the same gamete. Remember that at the end of meiosis youhave separated these gene forms into a different gamete. Each egg or sperm carriesonly one. For polygenic traits (ones in which two or more genes work together to createthe phenotype) such as eye color (E + C genes, each with dominant and recessivealleles), list ALL the gamete combinations you can produce.

The following are illustrations to help you determine phenotypes and genotypes.(Phenotypes are what you see, such as a "rounded or squarish" face. Genotypes are thealleles, such as "RR" or "Rr" or "rr.")

1. Face shape (compare cheekbone width and jawbone width)Rounded (RR or Rr square (rr

2. Chin prominenceProminent (CC or Cc) less prominent (cc)

3. Chin shapeRound (RR or Rr square (rr


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4. Cleft chinNo cleft (CC or Cc) cleft (cc)

5. Ear size (incomplete dominance)Large (EE) medium (Ee) small (ee)

6. Ear lobesFree (FF or Ff) attached (ff)

7. Hairy ears (sex-linked on Y chromosome)This affects the child ONLYif he is male. This gene is on the Y chromosome.If the father has hairy ears, the child will have hairy ears. Females dont havea Y chromosome. (Sorry, guys.)

Hair present (XYH) hair absent (XYh)


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Curly (CC) wavy (Cc) straight (cc)

9. Widows peakPresent (WW or Ww) absent (ww)

10. Hair color (polygenic)Several different genes interact to produce hair color. It is still not clearexactly how many genes are involved, but for this exercise, we will assumethat there are four genes (A/a, B/b, C/c. D/d). The more dominant allelesyou have, the darker your hair color.

8 dominant alleles = black 3 dominant alleles = dark blonde

7 dominant alleles = very dark brown 2 dominant alleles = blonde

6 dominant alleles = dark brown 1 dominant allele = very light blonde

5 dominant alleles = brown 0 dominant alleles = almost white blonde

4 dominant alleles = light brown

11.Eye size (incomplete dominance)


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Large (BB) medium (Bb) small (bb)

12.Eye shapeAlmond/wide (AA or Aa) round/narrow (aa)

13.Eye slantHorizontal (HH or Hh) upward slant (hh)

14.Eye distance (incomplete dominance)Close together (DD) medium (Dd) far apart (dd)

15.Eye lashes

Long (LL or Ll) short (ll)

16. Eye color (polygenic)


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Again, we have a polygenic trait for which the exact pattern of inheritance isnot clear. We will assume that two genes are involved. The first gene (E/e)determines the pigment at the front of the iris. The second gene (C/c)determines the color at the back of the iris.

EECC = almost black Eecc = gray-blue

EECc = dark brown eeCC = green

EEcc = brown eeCc = dark blue

EeCC = brown with green flecks eecc = light blue

EeCc = hazel

17. Eyebrow thickness (bushiness)Bushy (BB or Bb) fine (bb)

18.Eyebrow separationNot connected (GG or Gg) connected (gg)

19.Eyebrow color (incomplete dominance)Darker than hair (WW) same as hair (Ww) lighter than hair (ww)

20.Mouth length (incomplete dominance)


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Long (LL) medium (LL) short (LL)

21. Lip shapeThick (TT or Tt) thin (tt)

22. Protruding lip (Hapsburg lip) (incomplete dominance)Very protruding (HH) slightly protruding (Hh) absent (hh)

23. Lip color (incomplete dominance)Red (RR) rosy pink (Rr) light pink (rr)

24. DimplesPresent (DD or Dd) absent (dd)

25. Nose size (incomplete dominance)


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Big (NN) medium (NN) small (NN)

26. Nose shapeRounded (RR or Rr) pointed (rr)

27.Nostril shapeRounded (RR or Rr) pointed (rr)

28. Freckles on cheeksPresent (FF or Ff) absent (ff)

29.Freckles on forehead

Present (FF or Ff) absent (ff)

30.Skin color (polygenic)


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We know that several genes, perhaps as many as seven, contribute to skincolor. For this exercise, assume that there are three genes involved (A/a,B/b, C/c). The number of dominant alleles determines skin color (we use theunderside of the upper arm to determine skin color, since this area seldom isexposed to sun).

6 dominant alleles = very dark black5 dominant alleles = very dark brown4 dominant alleles = dark brown3 dominant alleles = medium brown2 dominant alleles = light brown1 dominant allele = fair0 dominant alleles = very fair

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How Did i Get This Face