Download Genetics in Longhorns

Exploring Genotypes and Phenotypes
of Longhorn Cattle

Phenotype – describes the physical appearance of a specific genetic trait or characteristic

Genotype – the genetic code consisting of a pair of alleles that describes a inheritable characteristic
or trait

DNA - a double helix chain of nucleic acid in a cell that carries genetic and hereditary information

Chromosomes – a strand of DNA that carries genes in linear order

Gene- a unit of inheritable information arranged located within chromosomes

Allele – one member of a pair of genes that determines genetic characteristics

Wild-type Allele – the gene or characteristic that most commonly occurs in the natural environment
this allele is identified as wild by a + symbol following it’s letter designation.

Heterozygous - a pair of alleles that contain two different alleles one of which is dominant

Homozygous – a pair of identical alleles

Dominant Trait – a trait that will appear in offspring if one allele is present . This trait will appear in
both heterozygous and homozygous gene pairs.

Recessive Trait – a trait that will appear only if two copies of the allele are present. This trait only
appears in homozygous gene pairs

Incomplete dominance – an allele that is not completely recessive to the dominant allele

Punnett Square – a square model of the allele genotypes used to predict the outcome of a genetic
cross.
 All cells contain DNA which is the “blueprint” that describes how an
animal will look or act. DNA the genetic information which
determines heredity. Genetics is the Science of ½’s. The genetic
information for a specific trait is contained in a pair of genes called
alleles. One half of the pair comes from each parent.
Offspring
Sometimes beef ranchers cross longhorn bulls with
their cows because longhorn calves are much smaller
than beef calves. Smaller calves mean that less cows and
calves die during the birthing process. However many
beef ranchers don’t want horns in their herd.
 The alleles in a the gene pairs can be the same homozygous
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or different heterozygous.
When they are heterozygous one allele will be dominant
and the other recessive.
The polled trait is described as HH. The horn allele is
described as hh.
The polled allele, H, produces no horns, H is completely
dominant over the recessive allele, h, that produces horns.
Any time the dominant H allele is present the animal will
not have horns.
Offspring receives
one allele from each
parent.
The combination of
these alleles makes
up the genotype
which determines
the phenotype of
each offspring.
Results :
100% of the offspring’s
genotype is heterozygous
type Hh.
100 % of the offspring’s
phenotype is polled, does
not have horns.
 We know that 100% of the offspring of a homozygous
crosses will not have horns. What happens when we
cross heterozygous crosses? Will any of the offspring
have horns. Fill out the next Punnett Squares to
predict the outcome of crossbreeding heterozygous
cattle.
What percentages of each genotypes resulted in this cross?
What percentage of each phenotypes resulted in this cross?
Were the phenotype and genotype results the same? Why or Why not?

Homogeneous crosses result in 100% of the offspring having the same genotype and
phenotype.

Heterozygous crosses have a variable result.
Genotypes:
HH - 25%
Hh - 50%
hh - 25%
Phenotypes :
Polled - 75%
Horns - 25%
Genotypes:
Hh - 50%
hh - 50%
Phenotypes :
Polled - 50%
Horns - 50%
A. Both of this calf’s parents had horns
B. This calf’s sire and dam both carry the recessive allele for horns.
C. The genotype represented by this calf is hh.
D. The genotype represented by this calf is either Hh or HH.
 All color in cattle is the result of two pigments black and red.
 Black can look brown in lower concentrations.
 Red can appear orange or yellow.
 White areas are a result of lack of both pigments.
 Three alleles control the amount of pigments in cattle ED, E+ and e
 The ED allele produces black pigment.
 The E+ allele is called the wild-type allele and produces both red and
black pigments. Calves are red at birth and turn dark brown or gray as
they mature, usually with a light muzzle.
 The e allele produces red pigment.
 The wild-type allele is thought to represent the ancestral coloration of
the wild Aurochs, from which modern Bos taurus cattle breeds have
descended.
Black
ED/ED
Dark Brown or Gray
E+/E+ (wild allele)
Red
e/e
The black allele is dominant over both wild and red alleles .
The wild allele is dominant over the red allele.
(ED > E+ > e)
Three Alleles make the base color genetics more complex list all
possible phenotypes under their genotypes. Possible Allele
Combinations: ED ED, E+ E+, e e, ED E+, ED e , E+ e
Fill out the Punnett Squares for all possible homozygous
color crosses.
How do the percentages in genotype results compare with phenotype results
in homozygous crosses?
Black Allele X Wild Type Allele
Genotype: 100% ED/E+
Phenotype: 100% Black
Wild Type Allele X Red Allele
Genotype: 100 % E+/e
Phenotype: 100 % Dark
Reddish Brown or
Reddish Gray
Black Allele X Red Allele
Genotype: 100 % ED/e
Phenotype: 100 % Black
Now try the following heterozygous crosses:
Two Alleles
Homozygous
X
Heterozygous
Same Genotypes
Heterozygous
X
Heterozygous
(ED > E+ > e)
List all possible phenotypes and genotypes and their percentages for each cross.
a. The cow and calf are both homozygous.
b. The calf inherited her color genes from her sire.
c. The cow is heterozygous and the calf inherited her
recessive gene.
d. This cow could not be this calf’s dam.
Three Alleles
Homozygous
X
Heterozygous
Different
Genotypes
Heterozygous
X
Heterozygous
 Seven pairs of alleles control the patterns of color
distribution.
 The pairing or combination of some of these genes creates
additional patterns
 Pigment reducing genes result in lighter variations of the
pigment and pattern alleles.
 All colors and patterns including the roans, spots, brindles,
speckled patterns, linebacks, grullas, reds, yellows,
oranges, browns, and blacks are a result of pigment
concentrations and genetic patterns.
(Phenotypes below are a few examples of each color variations and possible genotypes)
Brindle Alleles Br> br
( linear streaks of light and dark color patterned over the base color.)
E+/E+, Br/br
Roan Alleles R/r+
(base color is mixed with evenly distributed white to give a faded appearance.)
ED/ED, R/r+
Dilution Allele DS > dS +
ED/ED, DS/DS
E+/E+ ,DS /dS +
r/r, DS/DS
Spotted and Lineback Alleles SP>S+>s
(The lineback phenotypes will appear to have a complete or broken line of white along their back and belly.)
e/e, s/s
Color-sided alleles Cs>cs+
(A pattern similar in appearance to line back where color appears on the back and belly.)
e/e, Cs/cs
Dun Allele s DN +> dN
r/r, DN +/dN
e/e, Cs/Cs
Brockling Alleles Bc>bc+
r/r, s/s, Bc/Bc
r/r, s/s, Bc/bc
Some allele pairs produce a even wider variety of patterns when combined with other pairs these
include: Brockling, Dilution / Dun, and Color Sided / Roan and Spotted Alleles
The combination of color alleles produces an enormous
variety of color possibilities.
Historical Author J. Frank Dobie wrote,
“The colors were more varied than those of the rainbow. …
The shadings and combination of colors were so various
that no two were alike" - J. Frank Dobie, "The Longhorns"
Geneticist study the longhorn for several reasons. Most
colors and color patterns found in all cattle breeds occur in
the longhorn. Unlike other domestic breeds of cattle
longhorns developed through a process of natural selection
in the 17th , 18th, and 19th centuries. Longhorn genetics are
also studied to establish genetic markers that separate it as
a breed.
 Dr David M. Hillis, ”The Genetics of Coloration in Texas Longhorns” , Parts I-V
, 2004, University of Texas, http://doublehelixranch.com/color.html
 Dr. David Kirkpatrick, “Color Inheritance in Beef Cattle”, Animal Science,
University of Tennesse,
http://animalscience.ag.utk.edu/beef/pdf/ColorInheritenceFDK2004.pdf